Abstract

Researches about comprehensive, systemic and multi-level assessment methods are limit. Therefore, a comprehensive assessment method of heat exchangers based on extension theory is proposed. Heat transfer, pressure drop, mechanism, safety and reliability, economy, structure, mechanism and production performance are considered and evaluated in detail. By using this method, the comprehensive performances of the shell-tube and printed circuit heat exchangers as the intermediate heat exchanger of the accelerator driven sub-critical system are evaluated for comparison. The results show that the economy performance and pressure drop for the shell-tube heat exchanger attain the highest grades because the processing technology is mature and flow velocity is lower. However, the structure performance parameter is the lowest level due to its large volume and mass. For the printed circuit heat exchanger, the safety and reliability performance attains the lowest grade due to its easily blocked mini-channels and the manufactured thin plate. However, the mechanism performance shows the highest grade. In addition, the pressure drop performance also attains the highest grade due to its large cross-sectional area and low flow velocity. In conclusion, the comprehensive performance of the printed circuit heat exchanger is theoretically better than the shell-tube heat exchanger.

Introduction

As a clean and renewable source of energy, nuclear energy has the unique advantage and development potential on reducing the emissions of greenhouse gases and mitigating the pressure of energy transmission effectively [1,2]. Currently, how to dispose the nuclear waste has the constraint on the development of nuclear energy. Through the collision of proton beam generated by the accelerator with the spallation target of lead-bismuth eutectic (LBE), the accelerator driven sub-critical system (ADS) could drive and sustain the process of the nuclear transmutation, and reduce the life and radioactive property of nuclear wastes, and therefore is one of the effective resolutions [3]. In this system, LBE is adopted as not only the medium of the spallation target but also the coolant of the reactor due to its good thermal-physical properties [[4], [5], [6]]. The heat produced by the reactor is transferred from LBE to the working fluid of the power generating system through the intermediate heat exchanger (IHE), and the performance of the IHE plays significant impacts on the functions of the ADS and the power generating system. Therefore, systematic and reasonable performance evaluation on the IHE is of great significance.

At first, the performance assessment on the IHE with LBE is from the first principle of thermodynamics. Heat transfer rate, heat transfer coefficient and pressure drop are directly adopted to present heat transfer and flow characteristics of heat exchangers. For example, Ma et al. [7] experimentally investigated the effect of the straight and U-type tubes on the performance of the IHE with LBE. The heat transfer coefficients and pressure drops are compared between these two heat exchangers. Xi et al. [8] experimentally tested the performance of a shell-and-tube heat exchanger (STHE) with LBE and helium. The heat transfer rate, the overall heat transfer coefficient and pressure drop of the heat exchanger are obtained with increasing the Reynolds number and inlet temperature. In order to increase commonality, parts of research investigate the dimensionless performance parameters related to heat exchangers, such as Nusselt number, friction factor, effectiveness and number of transfer units (NTU). Chen et al. [9] numerically simulated a STHE with LBE and they utilized the overall heat transfer coefficient to compute the NTU and effectiveness of the heat exchanger for evaluation. Wang et al. [10] compared the effectiveness and friction factor of the STHE with LBE and helium with those concluded from correlations. Khan et al. [11] designed a compact heat exchanger with LBE and the heat transfer coefficient and Nusselt number are adopted to evaluate the performance of the heat exchanger. In order to fully consider the flow and heat transfer characteristics of the IHE, a comprehensive performance evaluation criterion (PEC) is introduced. Zhao et al. [12] changed straight tubes with twisted tubes for the IHE. The results show that the PEC of the twist-tube heat exchanger is improved by 1.2–1.7 times compared with the straight-tube one. Yang et al. [13] numerically studied the STHE with LBE and water, and the PEC is analyzed to optimize the inlet velocity of LBE.

Conclusions

In this study, through the analysis on matter element model, a comprehensive evaluation system of heat exchangers is built. The classical and joint domains are confirmed according to the databases and expert opinion. The weights of the performance indexes of the heat exchanger are set based on fuzzy analytic hierarchy process. The correlated function and evaluation grade are defined. At last, a comprehensive evaluation method of heat exchangers based on extension theory is proposed.

The SHTE and PCHE are chosen as the IHE with LBE and helium of the ADS for theoretical comparison. Although the flow channel in the SHTE is larger and that in the PCHE, the pressure drop performance for these two heat exchangers show the same grade. This is because the cross-sectional area of the PCHE is large and therefore the fluid flow velocity is low. The mechanism performance of the PCHE attains the highest grade because the plates are welded by the vacuum diffusion bonding. In conclusion, these two heat exchangers show the same evaluation grade. However, the comprehensive performance of the PCHE is theoretically better than that of the STHE due to the larger characteristic value. With the develop of the processing technology and the wide application of the PCHE, the production cycle is shortened and the manufacture and cost is deduced. In the future, the comprehensive performance of the PCHE would be even better.

Abstract

Over 766 million people have been infected by coronavirus disease 2019 (COVID-19) in the past 3 years, resulting in 7 million deaths. The virus is primarily transmitted through droplets or aerosols produced by coughing, sneezing, and talking. A full-scale isolation ward in Wuhan Pulmonary Hospital is modeled in this work, and water droplet diffusion is simulated using computational fluid dynamics (CFD). In an isolation ward, a local exhaust ventilation system is intended to avoid cross-infection. The existence of a local exhaust system increases turbulent movement, leading to a complete breakup of the droplet cluster and improved droplet dispersion inside the ward. When the outlet negative pressure is 4.5 Pa, the number of moving droplets in the ward decreases by approximately 30% compared to the original ward. The local exhaust system could minimize the number of droplets evaporated in the ward; however, the formation of aerosols cannot be avoided. Furthermore, 60.83%, 62.04%, 61.03%, 60.22%, 62.97%, and 61.52% of droplets produced through coughing reached patients in six different scenarios. However, the local exhaust ventilation system has no apparent influence on the control of surface contamination. In this study, several suggestions with regards to the optimization of ventilation in wards and scientific evidence are provided to ensure the air quality of hospital isolation wards.

Introduction

The study of airborne diseases has recently become increasingly significant with the emergence of severe acute respiratory syndrome, Ebola virus, tuberculosis, coronavirus disease 2019 (COVID-19), and other infectious diseases (Hathway et al. 2011). With the worldwide outbreak of COVID-19 (as of May 10, 2023), there have been 765,903,278 people diagnosed with COVID-19, including 6,927,378 deaths globally (WTO 2023). Some countries or regions succeeded in preventing the spread of the epidemic by managing quarantines and closures, which seriously affected economic growth and the normal operation of whole cities. Over the last 3 years, countries have prioritized preventing and controlling the epidemic. At the beginning of the outbreak, 100,000 people died every day worldwide as a result of COVID-19. It can affect anyone anywhere, especially healthcare workers exposed to environmental existing virus droplets. Therefore, the study of airborne virus diffusion in hospital isolation rooms is significant.

Viral respiratory infections are diffused by contact and droplet transmission, fomite transmission, and other types of transmission (WTO 2020). Contact transmission occurs when a healthy man is in close contact with an infected person (direct contact) or when an infected individual exhausts fomites containing viral droplets (indirect contact) (Morawska and Cao 2020). Viruses spread through the air via droplets or aerosols produced during coughing, sneezing, and talking (Jones and Brosseau 2015). Coughing, which is an intense process of droplet generation over transient time, has recently been the focus of many studies. Small virus droplets expelled into the air can exist as airborne droplets for a long time (Chartier et al. 2009) and can fill the whole room under the effect of an air conditioner (Liu et al. 2020b). Li et al. 2020a, b) investigated the spread of droplets in a tropical outdoor environment and found that at wind speeds of 2 m/s, 100 μm droplets can move up to 6.6 m. Wei and Li (2015) modeled the coughing process as a turbulence jet to investigate the diffusion of droplets and obtained travel distances with different droplet sizes. Lindsley et al. (2016) compared aerosol droplet samples of 61 adult volunteer outpatients through experiments and found that more aerosol droplets containing viruses were released through coughing. Nicas et al.

Conclusion

In this study, the full-scale geometry of the isolation ward in Wuhan Pulmonary Hospital is modeled, and the droplet diffusion process from a coughing event in the ward is simulated. An experiment is conducted to validate the CFD model, and a local exhaust ventilation system is designed to ensure indoor air quality.

(1) Although the local exhaust ventilation system reduces the number of airborne contaminants, a substantial increase in turbulent air motions may occur. The results show that a negative outlet pressure of 2.5 and 3.5 Pa led to a decrease in evaporated droplets and airborne contaminants remaining indoors, and the negative outlet pressure of 3.0 Pa increases the number of droplets tracked in the ward of the excessively turbulent flow. It is found that the number of droplets remaining indoors on the outlet negative pressure is not linear. The exhaust system can effectively extract contaminants when the negative pressure value is small. However, the negative pressure is increased, and the local exhaust system promotes turbulent transport, leading to a complete breakup of the droplet cluster and enhancing the dispersion of droplets indoors. The mass of the droplets is light, and when the outlet negative pressure value reaches a certain value, droplets can also be effectively removed.

Abstract

Temperature is known to greatly affect the efficiency, security, and cycle life of lithium-ion battery (LiB) cells. LiB cells are delicate to changes in temperature by using a variety of nanofluids. This study uses a novel cooling system with a Re between 15 × 103 and 30 × 103 to lower the cells’ temperature. Al2O3, CuO, SiO2, and ZnO with nanoparticles concentrations of 5% and nanoparticle diameters of 20 nm dispersed in a base liquid (water) are used to produce the effects. The findings demonstrate that as the Re rises, so does the Nusselt number. An innovative cooling system is designed and numerically tested to show how different kinds of nanofluids affect the increment in heat transmission and distribution of temperature in LiB cells. The temperature of LiB cells drops by flowing the water between the 52 batteries inside the cooling pack. For each spacing value, the Reynolds numbers are increased, which results in an increase in the average Nusselt numbers. According to the computational fluids dynamics results, the Nusselt number rises with increasing spacing. The results show that Re of 18000, 22000, 25000, 27500, and >30000 are needed for SiO2 nanofluids, Al2O3 nanofluids, ZnO nanofluids, CuO nanofluids, and pure water, respectively, to get battery pack temperature of <40 °C (the typical operating conditions). Higher Re values improve the heat transfer slightly with the expense of pumping power; therefore, Re of 18000 with SiO2 nanofluids is preferred. The utilization of nanofluids also shows that SiO2 exhibits the best thermal cooling for battery packs among all investigated nanofluids. Al2O3 nanofluid is also an excellent option, followed by ZnO nanofluid. For example, the temperature reduction of the hottest battery cell reaches >47, 44, 43, 42, and 42 °C for SiO2 nanofluids, Al2O3 nanofluids, ZnO nanofluids, CuO nanofluids, and pure water, respectively.

Introduction

The Greater demands for battery thermal management systems (BTMS) have been made as lithium-ion batteries are increasingly used in solar systems, power electric cars, and any engineering applications People are most concerned with the temperature at which batteries operate, their reliability, their safety, and their cycle life. The fluid cooling system can manage the peak battery temperature and the temperature differential among batteries within a tolerable range, therefore increasing the cycle battery’s lifespan [[1], [2], [3]]. However, the operating temperature has a significant effect on the efficiency and life of lithium-ion batteries. Too high or too low an operating temperature can damage the battery’s cycle life, efficiency, dependability, and safety, as well as cause substantial degradation of its charge and discharge performance [4,5]; Sheng, Su, Zhang, Li et al., 2019; Z [6]. When a lithium-ion battery’s working temperature is raised from 25 to 50° Celsius, the capacity loss increases by 100% after 300 cycles. Lithium-ion (Li-ion) batteries, as a vital element of electric automobiles (EVs), have drawn considerable interest in recent years [7]. Due to their high energy and power density, long cycle life, and low self-discharge, Li-ion batteries are considered the best energy storage technology for electric vehicles in comparison to other types of rechargeable batteries.

Conclusion

This paper investigates an effective cooling process for a battery pack having 52 batteries in a staggered arrangement (5 rows and 21 columns) using four nanofluids (i.e., SiO2, Al2O3, ZnO, CuO) and pure water. The flow and energy equation is modeled using ANSYS Fluent, which is validated against reported experimental data. The nanofluids are taken at the same concentration (5 vol%) and particle size (20 nm). Based on the applied conditions, the following remarks are concluded:

– Higher Re values result in a better cooling process and reduce battery cells’ temperature. To get a typical battery pack temperature of <40 °C, Re = 18000, 22000, 25000, and 27500 for SiO2, Al2O3, ZnO, and CuO nanofluids, respectively, should be used compared to Re > 30000 for pure water.

– The hottest battery cell temperature could be dropped by 47, 44, 43, 42, and 42 °C for SiO2, Al2O3, ZnO, CuO, and pure water, respectively.

– SiO2 is preferred since it cools all battery temperatures by < 40 °C with lower Re (i.e., 18000).

Abstract

Introduction

Methodology

PV efficienc

Data collectio

Experimental setup

Results and discussion

Effect of temperature on the power output of PV panels and TEGs

Conclusions

Nomenclature

References

Abstract

     Many factors affect the efficiency of photovoltaic panels (PV), which convert solar energy directly into electrical energy. Among these factors, temperature is one of the most important one. While some of the radiation from the sun is converted into electrical energy, part of it emerges as heat energy. This causes the photovoltaic cells to heat up and reduce their electrical efficiency. Different methods are used in the literature to reduce the temperature in PV panels. In this study, in order to reduce the adverse effects caused by the high temperature in the PV panels, 30 Thermoelectric Generators (TEG) were applied to the back surface of the PV panel to increase the PV panel output power and to produce additional electrical energy. Energy and exergy analysis made on the data obtained from both PV panels in the climatic conditions of the installation site showed that the temperature of the PV panel is reduced, and the energy and exergy efficiency is increased with the TEG application. At the end of July, August, and September, when the experiments were carried out, an average of 8.4% more electrical energy was obtained from a single PV panel with TEG, compared to the standard PV panel. Our results suggests that combination of TEG with PV panels could significantly increase the electrical energy, especially when a series of PV panels are used together.

Introduction

     Recently, investments in electricity generation from renewable energy sources, especially solar and wind energy, have been increasing. However, when the performance of photovoltaic cells in the natural field environment is compared with the version of the standard test conditions, the efficiency of the PV panels decreases with the increase in temperature due to radiation (Bel Hadj Brahim Kechiche, Hamza, and Sammouda 2016). Since the energy that cannot be converted into electricity increases the photovoltaic surface temperature, it causes damage to the PV panel module cells and leads to reduced service life of the PV panels (Enescu and Spertino 2017). When the studies in the literature about reducing the heat losses in photovoltaic panels (PV) are examined; A water-cooled nozzle spray system has been developed that significantly reduces the module cell temperature (Benato and Stoppato 2019). The factors affecting the performance of PV panels and the applications that can be made to increase the panel power are explained in detail (Fouad, Shihata, and Morgan 2017).To obtain higher efficiency by cooling the PV panel surface, the panel output power and the cell’s lifetime are increased with different cooling methods (Siecker, Kusakana, and Numbi 2017). To prevent the decrease in PV panel power output with temperature increase, new models are proposed on the bottom surface. In experiments carried out under natural conditions, it has been determined that the panel temperature can be reduced by 10°C with the evaporative cooling model and the power improvement is 5% (Haidar, Orfi, and Kaneesamkand 2021).

Conclusions

     In this study, the losses in energy production due to the increase in temperature in the PV panels, especially in the summer months, were reduced by TEG application. An increase in the PV Panel output power was achieved. According to the results obtained;

● More electrical energy was produced in the PV panel with a thermoelectric generator in July, August, and September when the experiments were carried out.

● With the application of a thermoelectric generator in the PV panel, an average of 8.4% more electrical energy was obtained per month. This indicates that when the number of PV panels combined with TEG is increased, the total energy could increase significantly

● The output power of TEGs also increases depending on the rise in air temperature and PV panel surface temperature

● Since the average temperature was higher in July, maximum energy was produced in the thermoelectric generator panel. Compared to the standard panel, the PV panel, in which a thermoelectric generator is applied, had 10.29% more electricity in July, 7.15% in August, and 7.09% more in September, respectively.

Abstract

Abbreviations

References

Acknowledgements

Author information

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About this article

References

Abstract

In this paper, based on the Li-Peterson pumping consumption-thermal resistance optimization model, a single-phase structure-optimized large-scale microchannel heat sink with each channel having 0.2 mm width and 0.8 mm height for extremely high heat flux cooling was proposed and investigated. Employing deionized water as coolant, two different heat source areas were designed and the results were compared under different pumping power from 0.1 W to 6.5 W. The experimental and simulation results indicates that the proposed copper-based microchannel thermal management system can dissipate heat flux of 1000 W/cm2 over 1cm2 and 500 W/cm2 over 5cm2, respectively, adding critical data support to the database of single-phase microchannel heat sink with heat removal capacity exceeding 1000 W/cm2. Moreover, the possible minimum thermal resistance over a broad pumping power range of 0.1 W to 6.5 W was explored. Extremely low thermal resistance of 0.065 K/W and 0.019 K/W were obtained for these two heating area scenarios. Overall, the proposed copper-based optimized microchannel heat sink is an ideal solution to cool high heat flux devices.

Introduction

The heat fuxes of devices such as high-performance computer chips, lasers and nuclear reactor are rapidly increasing, and the problem of heat accumulation and overheating is getting deteriorate [1]. In recent decades, the heat fux of the chip has reached 500 W/cm2 in the feld of Micro-ElectroMechanical Systems (MEMS), and the local hot spot can exceed 1000 W/cm2 [2–5]. For instance, the thermal fux of the next generation of IGBT modules will gradually increase from 100 W/cm2 to 500 W/cm2 [6]. For high-power laser devices, each diode laser needs to dissipate 500 W/cm2 of heat fux while keeping the operating temperature as low as possible [7]. In the nuclear power engineering, the heat fux generated by components included in fusion reactors and defense applications can be up to 104 W/cm2 [8].

Generally, the operating temperature of electronic devices should be less than 85 °C, and the reliability of products will be decreased by 50% if over-heated temperature exceeding 10 °C [9–11]. Since the demand of miniaturized design also limits the heat dissipation space, the uneven internal temperature distribution and poor heat dissipation have a series of negative efects, which will ultimately afect the performance and reliability of electronic devices. Therefore, reasonable thermal management is critical to electronic products [12, 13].

Current cooling technologies mainly include air cooling, liquid cooling, heat pipe, micro structure and spray cooling. As proposed by previous works, the traditional cooling method has relatively limited heat dissipation capacity, while the microchannel liquid cooling can handle a heat dissipation load of more than 1000 W/cm2 [14–16], which is expected to meet the urgently demand for high heat fux cooling in the near future.

1- Introduction

2- Methodology

3- Results and discussion

4- Conclusion

References

Abstract

This paper presents the effect of heat exchanger design on heat pump performance based on partial load operating conditions. 3-D numerical analysis was conducted to calculate face velocity profiles for each outdoor heat exchanger design (rectangular, cylindrical, and trapezoidal) in 10 different operating conditions. Heat exchanger circuits were modified considering heat exchanger face velocity distributions, and seasonal heat pump performances were calculated with modified heat exchanger design. The maximum seasonal performance enhancement of 7.07% was achieved with a modified heat exchanger design. Air-side flow maldistribution could affect significantly refrigerant path design and heat exchanger performance as well as system performance. The analysis results also revealed that smaller refrigerant circuits at the upper part of the heat exchanger interacting with higher air velocity could further enhance the annual system performance.

Introduction

Energy and environmental issues are critical problems due to rapid increases in energy systems around the world. The International Energy Agency has reported that energy systems such as air conditioning and heat pump systems account for almost 700 million metric tons of CO2 equivalent direct (7%–19%) and indirect emissions (74%) per year, which is responsible for global warming and ozone depletion. Direct emissions can be reduced by introducing low global warming potential (GWP) refrigerants, and indirect emissions can be controlled using a more efficient system [1]. The finned tube heat exchanger is widely used as a condenser or evaporator of heat pump systems, and it is one of the most important components of the systems and plays a significant role in the system size and performance. Therefore, it is important to design more efficient, lightweight, cost-effective, and low power consuming finned tube heat exchangers [2,3]. Heat exchanger performance degradations are associated with maldistribution of refrigerant and face air velocity distribution, particularly for downstream rows as airflow velocity decreases significantly on the lower side [4]. Although it is difficult to measure the air velocity profile experimentally [5], it strongly influences heat exchanger performance [6]. Non-uniform flow distribution has many disadvantages: produces wake in flow [7], changes flow pattern [8], decreases overall heat transfer coefficient [9], and reduces system capacity [10]. Consequently, it also increases pressure drop [11] across the heat exchanger hence increases required pumping power [12–15]. Performance deterioration due to non-uniform flow is not limited to finned tube heat exchangers with louver fins [9] and plate fins [16,17], but also occurs in concentric tube [14], and microchannel condensers of automotive air conditioning systems [18]. These examples highlighted the problem of non-uniform flow distribution in different applications. However, these studies are limited to rated load conditions for specific heat exchanger design. No study has been found in literature that covers face velocity distribution for outdoor heat exchanger under partial load operating conditions. Therefore, it is important to evaluate face velocity profiles under off-design condition.

1- Introduction

2- Experimental investigation

3- Procedure and Calculations

4- Results and discussion

5- Conclusion

References

Abstract

Nanofluids have attracted huge attention because of their effective physical and thermal properties. One of many applications of nanofluids is the enhancement of the thermal performance of heat exchangers. In the current study, an experimental investigation has been conducted for studying the effects of graphene nanofluids on the convective heat transfer in a vertical shell and tube heat exchanger. Graphene flakes were prepared using graphite foam that is derived from sugar as a raw material. The prepared Graphene flakes have been characterized using scanning electron microscopy, X-ray diffraction, atomic force microscopy, and Raman spectroscopy. The graphene nanofluid has been used in the tube side of the heat exchanger to enhance its heat transfer performance. Different parameters such as nanofluids’ concentration, flow rate and inlet temperature were studied and their effects on heat transfer coefficient and thermal efficiencies are discussed. The results show that using of graphene/water nanofluids enhances the thermal performance of the vertical shell and tube heat exchanger. A maximum increase in the heat transfer coefficient of 29% was achieved using 0.2% graphene/water nanofluids. Furthermore, the mean thermal efficiency of the heat exchanger was enhanced by 13.7% by using graphene/ water nanofluid.

Introduction

Heat exchangers are widely used by various types of industries to exchange heat between different fluids for waste heat recovery and utilities cost reduction. The thermal and physical properties of the heat transfer fluids are crucial factors that dedicate the efficiency of heat exchangers. During the last two decades, nanofluids have attracted a huge attention of researchers due to its enhanced thermal properties and flow characteristics [1]. These advantages make nanofluids promising heat transfer fluids to be used for heat transfer enhancement. Many experimental studies have been conducted for the evaluation of heat transfer characteristics of nanofluids in different types of heat exchangers [2–7]. These types of heat exchangers include plate heat exchangers [2], double pipe heat exchangers [3], and micro heat exchangers [6]. However, in open literature, similar studies on shell and tube heat exchangers are scarce. Thermal performance of a shell and tube heat exchanger was analytically investigated by Shahrul et al. using nanofluids of four different types of nanoparticles, Fe3O4, ZnO, TiO2, CuO, and Al2O3 [8].

Gas-gas plate heat exchanger is an important component to remove heat generated from electronic devices in the cabinet cooling system. The counter-flow plate heat exchanger is usually used due to its higher heat transfer performance than the cross-flow plate heat exchanger. However, in the cabinet cooling system, the overall dimensions for the heat exchanger is limited. Therefore, it is necessary to consider the overall dimensions of the system during heat exchanger design, but the traditional thermal design method of heat exchanger doesn’t consider the effect of system parameters. In this paper, a cross-flow plate heat exchanger is proposed to improve the cooling performance of cabinet cooling system, and is compared with the counter-flow plate heat exchanger. The ε-NTU method and effectiveness-thermal resistance method are applied to evaluate the performance. It is found that for large width of cabinet cooling system, the system with cross-flow plate heat exchanger has higher cooling performance and lower thermal resistance than the system with counter-flow plate heat exchanger. When the width is 700 mm, the cooling capacity of the two systems are 176.13 W/K and 138.95 W/K, respectively. The dimensionless thermal resistance can characterize the irreversibility of the heat transfer at constant mass flow rate.

Introduction

Plate heat exchangers are widely used in energy consuming and handling industries [1]. Generally, plate heat exchangers are used for liquid–liquid heat exchange [2]. Imran et al. [3] optimized liquid–liquid plate heat exchanger. The overall dimensions of the heat exchanger were independently considered in their research. Miao et al. [4] numerically studied liquid–liquid plate heat exchangers. The heat exchanger was counter-flow plate heat exchanger. Two methods were utilized to get simulation results, respectively. The results of grey-box method were better. Kumar et al. [5] experimentally studied the liquid–liquid plate heat exchangers and obtained correlation based on their experimental data, in which the overall dimensions of the measured heat exchangers were constant. Yang et al. [6] obtained correlation based on their experimental data and empirical correlations in open literature. The overall dimensions of heat exchangers in their work were varied. Counter-flow plate heat exchanger also could be applied for two-phase heat exchange [7] and the heat exchangers in organic Rankine cycle could be counter-flow plate heat exchangers for better performance [8]. However, due to the primary heat transfer surfaces and light weight the plate heat exchanger was developed for gas–gas heat exchange [9] and even for liquid–gas heat exchange [10]. Cross-flow arrangement was convenient for liquid–gas plate heat exchanger [11]. High temperature gas–gas heat exchangers were mainly applied for the microturbine recuperated cycle system [12] and air preheater of the fossil fuel power system [13]. Recuperators for micro gas turbine was reviewed by Xiao et al. [14] recently. Wang et al. [15] improved the performance of air preheater by optimizing geometrical parameters of primary surface.

1. Introduction

2. Experimental apparatus and procedure

3. Modeling and calculation methodology

4. Experimental results and discussion

5. Conclusion

Acknowledgments

References

Abstract

Freeze concentration (FC) by suspension crystallization is a complex process due to the combination of a scrapedsurface heat exchanger, crystallizer, and wash column. Following the study of a three-in-one structure of a multipass FC published earlier this year, modeling and experiments of the heat and mass transfer of this freeze concentrator are presented in this paper. The experimental assessment of the system performance, including the measured values of the heat transfer coefficient, ice production rate, and energy efficiency as well as their correlation with the concentration ratio and partition coefficient are presented in this article.

Introduction

Freeze concentration (FC) is a nonthermal processing technology of liquid food, in which a portion of the water in the aqueous solution is frozen and converted into relatively pure ice crystals, after which it is removed from the liquid phase to concentrate the remaining solution. It can be used to concentrate or pre-concentrate heat-sensitive aqueous solutions, such as milk (Habib and Farid, 2007; Sanchez et al., 2011), fresh fruit juices (Orellana-Palma et al., 2017; Petzold et al., 2013; Bayindirli et al., 1993), other liquid foods (Moreno et al., 2014a), and biological solutions (Moreno et al., 2014b). Studies have shown that compared with evaporation concentration, FC has the advantage of producing less thermal denaturation of the solution, and thus, it can better maintain the original flavor, nutrition, and color of liquid foods (Benedetti et al., 2015; Moreno et al., 2014c; Miyawaki et al., 2016a). Moreover, the latent heat of water freezing is almost one seventh of the latent heat of water evaporation (Lide and Haynes, 2010), which offers potential for energy savings for de-watering of aqueous solutions.

Ice crystals can be formed from aqueous solutions in two ways: progressive crystallization (Miyawaki et al., 2005, 2016b; Zambrano et al., 2018) and suspension crystallization (Huige and Thijssen, 1972; Qin et al., 2007). In the former, the water freezes on the cooling surface, forming an ice layer progressively and a concentrated liquid phase. Thus, it is also known as layer crystallization. This method of FC is called progressive FC. When the ice layer extends to the entire vessel to form an ice block, it is known as block FC (Moreno et al., 2014c; Zambrano et al., 2018). The advantage of this technology is the low equipment cost and simple operation management. However, the ice layer has a poor heat transfer coefficient of less than 0.1 kW m 2 K 1 (Qin et al., 2003a; Pronk et al., 2010; Hasan et al., 2017), and a huge cooling surface area is required for practical applications. In addition, the ice layer tends to entrain liquid sacs and causes severe solute loss (Miyawaki et al., 2016a, 2016b; Samsuri et al., 2015).

1. Introduction

2. Materials and experimentation details

3. Results and discussion

4. Conclusion

Acknowledgements

References

Abstract

Ceramic based nanoparticles are having great potential to improve the thermo physical properties of nanofluids for heat transfer applications. Hot Titanium grade-9 (Ti3Al2.5V) plate is quenched in two different fluids, namely aluminium oxide (Al2O3)/water nanofluid (NF) and aluminium oxide – titanium oxide (Al2O3-TiO2)/water hybrid nanofluid [HyNF] to remove high heat flux. The performance of nanofluids was analyzed through cooling rate determination. High energy planetary ball milling technique was used to prepare Al2O3 and TiO2 nanoparticles. A K-type thermocouple was embedded on the bottom surface of the plate to measure the temperature. The time–temperature data were recorded by the help of a data acquisition system. Experimental results revealed that the nanofluid as well as hybrid nanofluid surprisingly enhanced the cooling efficiency of titanium plate. Al2O3-TiO2/water hybrid nanofluid shows significantly higher value of cooling rate (around 15%) as compared to the conventional cooling fluids. Also hybrid nanofluid exhibit marginally higher efficiency compared to Al2O3/water nanofluid.

Introduction

Cooling is one of the foremost necessary technical challenges faced by industries such as automobiles, manufacturing and electronics. New technological developments are required for quicker cooling of hot surfaces. In materials science, quenching is the quickest cooling technique of a work piece in a medium such as water, oil or air to attain specific material properties. Quenching has a high cooling potential and is one of the effective cooling techniques specifically in processes such as steel manufacturing, metallurgy, microelectronic device making and thermal management. Nanofluids are new type of quasi single phase medium containing stable colloidal dispersion of nanometer sized metallic or ceramic particles, fibers, wires, rods, sheets or droplets in base fluids include water, organic liquids, oils and lubricants, bio-fluids, polymeric solutions and other general liquids. These nanofluids have great potential to enhance heat transfer for various applications [1–5]. Huang et al. investigate the effect of hybrid nanofluid mixture in plate exchanger for enhanced heat transfer characteristics [1]. L.S. Sundar et al. indicated that hybrid nanofluids are more effective heat transfer fluids than single nanoparticles based nanofluids or conventional hybrid fluids [5]. The review also demonstrates that hybrid nanofluids were exhibiting higher thermal conductivity than individual nanoparticle (Fe3O4) based nanofluid. The thermal properties, such as thermal conductivity of hybrid nanofluids exhibit higher values compared to single nanofluid. Kim et al. conducted detailed analysis of nanofluids exhibiting significantly higher nucleate boiling critical heat flux compared to pure base fluid [6]. The characteristics of critical heat flux augmentation in nanofluids were examined according to the properties of boiling parameters. Efforts to reveal the key factors prominent to nanofluid critical heat flux augmentation are summarized.

1. Introduction

2. Literature survey on existing database and correlations of heat transfer coefficient for upward no-phase-change two-phase flow in inclined pipes

3. Development of heat transfer multiplier correlation for upward two-phase flow in inclined pipes

4. Performance evaluation of the newly-developed two-phase heat transfer coefficient correlation for upward two-phase flow in inclined pipes

5. Conclusions

Acknowledgement

Appendix

References

Abstract

This study aims at developing a robust and theoretically-supported correlation of two-phase heat transfer coefficient for upward no-phase-change two-phase flow in inclined pipes based on the concept of extended Chilton-Colburn analogy. Firstly, a comprehensive literature survey was conducted to gather over 1800 experimental data and 12 correlations of two-phase heat transfer coefficient. The comparison results indicated that none of the existing correlations could predict the entire database satisfactorily. Then, the dependence of twophase heat transfer enhancement ratio (or two-phase heat transfer multiplier) on liquid fraction, two-phase pressure drop multiplier and inclination angle was analysed, and the two-phase heat transfer coefficient correlation was developed. The performance assessment indicated that the newly-developed correlation could predict 95% of the experimental data within ± 30% error with the mean absolute relative deviation of 12.9%. The newly-developed semi-theoretical correlation would be useful in designing no-phase-change two-phase heat transfer systems, such as petroleum pipelines and nuclear power plants.

Introduction

Upward no-phase-change two-phase flow in inclined pipes has extensive industrial and engineering applications in petroleum pipelines, chemical reactors and nuclear power plants [1]. Various flow patterns such as bubbly flow, intermittent flow, stratified flow and annular flow, occur in inclined pipes depending on different flow conditions [2]. Due to the significant interaction between gas and liquid phases, the flow characteristics of upward no-phase-change two-phase flow in inclined pipes are complicated. The thermal characteristics of no-phase-change two-phase flow in inclined pipes is attracting more and more attention recently. Substantial efforts have been made on understanding the flow and heat transfer characteristics of upward no-phase-change two-phase flow in inclined pipes.

Hetsroni et al. [3] investigated the local heat transfer coefficient of upward air-water flow in inclined pipes and claimed that the pipe inclination enhanced two-phase heat transfer. Mosyak and Hetsroni [4] also measured the temperature difference between the top and bottom of a horizontal pipe and slightly inclined pipes, and found that increasing pipe inclination could drastically reduce the temperature difference. Vaze and Banerjee [5] studied the effect of pipe inclination on flow and heat transfer characteristics of air-water two-phase flow experimentally. Trimble et al. [6] conducted experimental study on heat transfer characteristics of upward two-phase slug flow in inclined pipes.

This paper presents a review of the contributions on entropy generation of nanofluids and hybrid nanofluids in the different types of thermal systems for different boundary conditions and physical situations . The relevant paper s are classified into three categories: entropy generation in minichannel, entropy generation macrochannel and entropy generation in cavities. The viscous dissipative, streamwise , electromagnetic effects, as well as nanoparticles concentration, the temperature and the flow regime on entropy generation , were analyzed. The reviewed literature indicates that the implementation of nanofluids/hybrid nanofluids in microchannels, minichannels , and cavities may be an important alternative to the traditional thermal systems and an interesting topic of study.

Introduction

Micro/nano heat transfer has been an important issue for researchers in the last decades. The microdevices (devices with dimensions between 1 ?? and 1 mm) are successfully employed in a variety of fields such as electronic s, aerospace , telecommunication, biomedical and automotive industries to enhance heat transfer for heating and cooling . The heat transfer efficiency can be improved by several methods, such as the use of extended surfaces, the application of vibration to heat transfer surfaces, and the use of mini/microchannels. Also, heat transfer efficiency of the microdevices can also be improved by enhanced thermo -physical properties of the working fluid, especially the thermal conductivity and the specific heat [1]. The working fluids with the thermo – physical properties improved are called nanofluid/hybrid nanofluid . By combining nanofluid/hybrid nanofluid with the small channel’s dimensions of the thermal devices, it is to obtain devices that provide compactness, low thermal resistance and efficiency. The entropy generation analysis is a useful tool for performance optimization of the thermal systems. It is known as the addition of nanoparticles into the base fluid could be influence the total entropy generation. Thus , the use the nanofluids in the thermal systems decrease the temperature of the system and finally the heat transfer contribution to the total entropy generation rate decreases, while nanoparticles added into the base fluid increase the viscosity of the working fluid leads to increase pressure drop in the system. Many researchers have studied the entropy generation in order to find optimum conditions for different thermal systems and only a few reviews have been devoted to this topic.

Targeting the heat transfer performance of geothermal pile-foundation heat exchanger in ground source heat pump system (GSHP), the physical models of 3-U pile-foundation heat exchanger and heat exchanger group were established. CFD software was used to simulate heat transfer processes and heat transfer performance was analyzed both in cooling and heating mode. The simulation results indicated that the higher thermal conductivity of pile-foundation heat exchanger contributed to the higher heat transfer efficiency than soil. Heat transfer flux per meter of the pile-foundation heat exchanger gradually decreased with time went on. After operating for ten years, the average soil temperature increased by 2.96 K in non-equilibrium condition and decreased by 0.61 K in equilibrium condition. The equilibrium condition of cooling and heating load was beneficial to operation system’s safety and efficiency. The experimental values of temperature differences were 2.2 K, 2.5 K and 3.5 K, and the heat transfer flux were stable at 58.1 W·m − 1, 65.9 W·m − 1 and 46.2 W·m − 1 in three schemes separately. The maximum difference value was 8.4% for temperature difference between experiment and simulation. The simulation results corresponded well with experimental data, indicating the reliability of simulation. The study results were approximate to the actual situation and can be used as theoretical basis for design and application of pile-foundation heat exchanger in GSHP system.

Introduction

With the development of society and improvement of people’s living standards, energy consumption has been growing apace. Nowadays, urban heating has been accounting for the largest proportion of the total building energy consumption in China [1]. For the purpose of saving energy and protecting environment, geothermal energy, as one of renewable energies, has attracted increasing interest around the world. One of the main technologies of using geothermal energy is ground source heat pump system (GSHP), which has been widely studied and applied since the early 20th century. In contrast to traditional air conditioning system, GSHP system has a lot of significant advantages. By extracting heat into and injecting heat from soil, GSHP system can operate stably by avoiding the impact of weather changes on system performance, meeting the requirements of sustainable devel-opment strategy. Without material exchanges with atmosphere, the pollution of waste heat, vapor and noise can be reduced. Moreover, GSHP system has a great advantage in investment and maintenance costs [2]. However, the disadvantages of GSHP system cannot be ignored neither. To invest a new system, a large area of land is needed and drilling holes costs additional investment. In 1994, steel pipes in pile foundation were applied to buildings for the first time, and then the concept of pile-foundation heat exchanger was proposed [3]. Consequently, geothermal pile-foundation heat exchangers in GSHP system with pipes laying inside the foundation and fixing via concrete are applied, contributing to declining outlays, saving ground area and reducing thermal interference between heat exchanger [4]. Compared with horizontal pile-foundation heat exchanger, vertical heat exchangers had better performance in heat transfer efficiency and land consumption [5].