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

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

The heat transfer performance (HTP) of an improved elastic tube bundle (IETB) was analyzed using a bi-directional fluid-structure interaction calculation method. The vibration-enhanced HTP of heat exchangers with different inlet velocities (Uin) and row number N were studied. The results show that the amplitude gradually increases as the Uin increase with a maximum increase of 309.98%. The amplitude with fixed shell side length (FL-heat exchanger) is generally higher than that with variable shell side length (VL-heat exchanger) under different row numbers. The average amplitude of the three directions increased by 4.55%, 11.54%, and 7.41%, respectively. The heat transfer coefficient (h) is directly proportional to Uin, and the comprehensive HTP gradually decreases with the increase of Uin. With the increase in the row numbers, h generally showed a downward trend. For the VL-heat exchanger, h decreases as the row number increases, which increases from N=6 to N=9, and h decreases by 13.85%. For the FL-heat exchanger, h first increases and then gradually decreases. The comprehensive HTP also maintained the same trend. Compared VL-heat exchanger and FL-heat exchanger, reducing the spacing between tube rows increases heat transfer per unit volume, leading to improved comprehensive HTP of the IETB heat exchanger.

Introduction

Heat exchangers [1–3] are commonly used devices for exchanging energy in industries such as aerospace, chemical, and refrigeration production. When the traditional tubular heat exchanger works, its internal rigid heat transfer elements begin to vibrate under the impact of fluid, which can result in fatigue damage and decreased heat exchanger service life [4,5]. Shell and tube heat exchangers [6,7] are currently the most widely used in industrial production. The study of enhanced heat transfer in heat exchangers has become a hot topic of current research [8]. To reduce the influence of fluid shock, utilizing the vibration-enhanced heat transfer principle, an elastic tube bundle heat exchanger is proposed [9,10]. The material of the heat transfer element was replaced from steel to copper to achieve small amplitude vibration of the internal tube bundle at the low frequency, this improvement not only improves the heat transfer coefficient (h), but also uses the vibration deformation to remove the scale on the heat transfer surface, reduce the thermal resistance, and finally realize the composite heat transfer enhancement. However, due to the disadvantages such as poor comprehensive heat transfer performance (HTP) of elastic tube bundles, it is essential to improve the structure to obtain better comprehensive HTP.

Conclusion

Based on the bi-directional fluid-structure interaction calculation method, the effect of Uin and row number N on vibration and HTP of the IETB heat exchanger are studied. According to the results above, the key conclusions follow.

(1) The average amplitude of IETBs in different row numbers gradually increases with the rising of Uin. The vibration uniformity of the IETBs is better at low Uin. Besides, the difference in amplitude among different IETBs becomes larger and larger. And the amplitude in the FL-heat exchanger is generally higher than the VL-heat exchanger. For the VL-heat exchanger, when the row number is increased, the average amplitude decreases gradually. For the FL-heat exchanger, it decreases first and then increases.

(2) The average heat transfer coefficient ha/hva of IETBs is directly proportional to Uin and comprehensive HTP decreases gradually with the rising of Uin. With the increase of tube number i, the ha/hva presents a general trend of decline. And ha/hav in the FL-heat exchanger is generally greater than that VL-heat exchanger. In addition, the hva is always greater than h, which indicates that vibration facilitates the achievement of heat transfer enhancement.

Abstract

Molten salt is one of the most promising heat transfer and thermal energy storage medium for the third-generation solar thermal power generation, and the spiral-wound tube heat exchanger can improve the heat transfer performance of molten salt. In this paper, the flow and heat transfer characteristics of molten salt nanofluids in spiral-wound tubes are experimentally and numerically studied. Molten salt nanofluids and synthetic oil as the heat transfer fluids, the heat transfer performance of them in the spiral-wound tube heat exchanger were studied. The Wilson separation method was used to process the experimental data, and the heat transfer correlation of molten salt nanofluids is obtained through experiments. The heat transfer correlation can be in good agreement with the experimental data, the heat transfer correlation on the molten salt nanofluids side is  , Re = 4296–8348, Pr = 13–23, and the maximum error is 17.9%. A three-dimensional model of the spiral-wound tube heat exchanger is used to explore the distribution of the pressure, temperature and velocity fields of the molten salt nanofluids. The friction factor correlation on the molten salt nanofluids side is  , Re = 3536–17684, Pr = 13–23 and the maximum error is 12.65%. The heat transfer correlation and resistance correlation of molten salt nanofluids obtained in this paper can contribute to the design of heat exchanger for the third-generation solar thermal power.

Introduction

Solar thermal power generation technology has received extensive attention due to high efficiency and sustainability. Energy storage technology is an effective measure to solve the problem of intermittent solar power generation [[1], [2], [3]]. Molten salt can be used in the field of energy storage because of its large heat capacity, low viscosity, low vapor pressure, scholars around the world have paid extensive attention and carried out related research [[4], [5], [6]].

Liu et al. [7] conducted an experimental study on the turbulent heat transfer of molten salt in a circular tube, and obtained the heat transfer correlation of molten salt in the circular tube. Chen et al. [8] conducted an experimental study on the convective heat transfer of Hitec salt in three kinds of transversely corrugated tubes. He et al. [9] explored the convective heat transfer characteristics of ternary salts in shell and tube heat exchangers through experiments. Yang et al. [10] studied the flow and heat transfer characteristics of molten salt in trough solar collectors by numerical simulation. Chen et al. [11] carried out numerical simulation on the convective heat transfer of molten salt mixed in a unilaterally heated horizontal square tube. The results show that under the condition of non-uniform heating, the buoyancy force causes the core area of molten salt flow to be close to the heating surface to form eddy currents, which can enhance the heat transfer effect. Yang et al. [12] studied the heat transfer and flow performance of molten salt in the annular channel with helical coils by numerical simulation. The results show that adding helical coils can effectively enhance the heat transfer of molten salt in the annular channel, but the flow resistance also increased. Du et al. [13] designed a U-shaped tube for the experimental test of the heat transfer characteristics of molten salt in a shell-and-tube heat exchanger in transition flow, and fitted the heat transfer correlation of molten salt. Qiu et al. [14] proposed a baffle rod shell-and-tube heat exchanger configuration applied in the field of concentrated solar thermal power generation, using ternary salt and heat transfer oil as shell-side working fluids for experimental research. He et al. [15] studied the turbulent heat transfer characteristics of molten salt in shell-and-tube heat exchangers with Reynolds numbers ranging from 10,000 to 91,000 and 11,000 to 27,000 on the tube side and shell side, respectively. The comparison of available correlations for molten salt is in Table 1.

Conclusions

In this paper, the spiral-wound tube heat exchanger is taken as the research object, and the molten salt nanofluids and synthetic oil are used as heat transfer fluid to carry out experiments and numerical simulation studies respectively. The experiment was carried out with the salt-side temperature ranging from 573 K to 613 K, mass flow rate ranging from 0.6 to 1 kg/s. The corresponding Reynold’s number range is Re = 4296–8348. The flow heat transfer characteristics inside the heat exchanger, the following results are obtained:

Abstract

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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.

Abstract

Keywords

1. Introduction

2. Derivation of the model

2.1. Original domain

2.2. Construction of the reference domain

2.3. Navier-Stokes equations

2.4. Asymptotic analysis

3. A new generalized lubrication model

4. A new thin fluid layer model

5. Conclusions

Appendix A. Change of variable

Appendix B. Coefficients definition

Appendix C. Derivation of equations to calculate

References

Abstract

In this paper we study the behavior of an incompressible viscous fluid moving between two very close surfaces also in motion. Using the asymptotic expansion method we formally justify two models, a lubrication model and a shallow water model, depending on the boundary conditions imposed. Finally, we discuss under what conditions each of the models would be applicable.

1. Introduction

The asymptotic analysis method is a mathematical tool that has been widely used to obtain and justify reduced models, both in solid and fluid mechanics, when one or two of the dimensions of the domain in which the model is formulated are much smaller than the others.

After the pioneering works of Friedrichs, Dressler and Goldenveizer (see [28] and [30]) the asymptotic development technique has been used successfully to justify beam, plate and shell theories (see, for example, [43], [16], [17], [15], [5], [54], and many others).

This same technique has also been used in fluid mechanics to justify various types of models, such as lubrication models, shallow water models, tube flow models, etc. (see, for example, [25], [24], [18], [3], [37], [55], [36], [31], [6], [2], [29], [26], [32], [33], [9], [23], [45], [46], [47], [48], [49], [50], [21], [22], [34], [35], [40], [41], [42], [10], [11], and many others).

In this work, we are interested in justifying, again using the asymptotic development technique, a lubrication model in a thin domain with curved mean surface. Following the steps of [3], but with a different starting point, we devote sections 2 and 3 to this justification. During the above process we have observed that, depending on the boundary conditions, other models can be obtained, which we show in section 4. In this section we derive a shallow water model changing the boundary conditions that we had imposed in section 3: instead of assuming that we know the velocities on the upper and lower boundaries of the domain, we assume that we know the tractions on these upper and lower boundaries.

Thus, two new models are presented in sections 3 and 4 of this article. These models can not be found in the literature, as far as we know. In addition, the method used to justify them allows us to answer the question of when each of them is applicable. In section 5 we discuss the models yielded, as well as the difference between one model and another depending on the boundary conditions, reaching the conclusion that the magnitude of the pressure differences at the lateral boundary of the domain is key when deciding which of the two models best describes the fluid behavior.

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].

1. Introduction

2. Materials and methods

3. Results

4. Conclusions

Appendix A. Supplementary data

References

Abstract

The thermal system design strongly depends on material selection. Nanofluids offer design flexibility and finetuning of properties by incorporation of nanoparticles in base fluids. This flexibility is guided by particle-particle communication, which may be beneficial in creating ballistic routes in heat transfer but also detrimental due to affecting nanofluid properties. The transition of nanofluids to industrial use requires application-based examinations. For this purpose, different types of nanofluids were investigated in this work in terms of their thermal efficiency in a flat plate solar collector (FPSC) and some figure-of-merits (FOMs), under laminar and turbulent flow conditions. Investigation of both aims at clarifying the correlation between FOMs and FPSC thermal efficiency, and further reporting on the validity of FOMs in assessing thermal efficiency. Results indicate that nanofluids’ eligibility as a heat transfer fluid depends on the flow condition, since a base fluid could outperform a nanofluid under turbulent flow. Nanofluid type and nanoparticle shape affects thermal performance, as suspensions of nanoplatelets/nanotubes in low concentrations (< 0.04 vol%/0.25 vol%) are shown to outperform certain spherical metal-oxide nanoparticles (< 3 vol%), according to some FOMs. It is shown that performance evaluation criteria (PEC), overall energetic efficiency, and energy ratio (ER) do not capture FPSC thermal efficiency trends, e.g., for graphene nanoplatelet nanofluid, as Mouromtseff number-based comparisons do for laminar and turbulent conditions. It must be highlighted that the FOM type to indicate thermal efficiency should be chosen depending on the application, and simultaneous consideration of thermal and hydrodynamic characteristics is required.

Introduction

In today’s world, the main emphasis in energy industry lies on the concept of “sustainability” in energy applications. Plenty of Rooms at the Bottom are available, as stated by Richard P. Feynmann [1], at the enhanced surface-to-volume ratio of nanoparticles which bring about unique characteristics to base fluids when mixed with nanoparticles. Nanofluids [2] or nano-enhanced Heat Transfer Fluids (ne-HTFs) have been under research in a variety of heat transfer-based systems. This has been by necessity due to the poor thermal characteristics of most conventional HTFs, e.g., water, ethylene glycol, mineral oils, brines, etc. compared to solids. This fact highlighted the need for their replacement by ne-HTFs as a potential way to improve thermal performance of solar collectors, electronic cooling systems, nuclear reactor cooling schemes, refrigerators, and so forth.

Nanofluids thermal conductivity and heat transfer coefficient have been among the most investigated characteristics [3–5] in determining ne-HTFs’ potential in various heating and cooling processes. Hydrodynamic and colloid state points of view show per contra that high viscosity of nanofluids cause increments in pressure drop, pumping power, operation cost, and nanoparticle sedimentation concerns to increase; as well as sometimes thermal performances no better than HTFs in turbulent flow and fully developed flow conditions [3,6–8]. Although the convective heat transfer coefficient and Nusselt number are important performance indicators, various types of figure-of-merits (FOMs) have been proposed to compare the heat transfer characteristics of HTFs. One of the most earliest FOM definition is the Mouromtseff number (Mo) [9] that has been used to compare heat transfer capabilities of HTFs in laminar and turbulent flow conditions. Another FOM is defined as the ratio of viscosity enhancement and thermal conductivity enhancement coefficients [10], valid in the case of fully developed laminar flow. Sekrani et al. [11] suggested two different types of FOMs to compare the heat transfer capability of nanofluids as the performance evaluation criterion, i.e., PEC, [12] and overall energetic efficiency.

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.