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.

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. 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. 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.

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].

Condensation heat transfer on biphilic surface is investigated. The surface periodically populates hydrophobic stripes each having a coating layer thickness δp and a width WDWC, and hydrophilic stripes each having a width WFWC. The proposed model includes dropwise condensation on hydrophobic stripe, filmwise condensation on hydrophilic stripe, and droplet detachment radius rmax criterion for heat-mass coupling between the two wettabilities regions. The rmax is the minimum of detachment radii determined by droplet removal modes of double-sides-suction DSS, one-side-suction OSS and sliding, where DSS is a special case of OSS for droplet located at hydrophobic stripe centerline. Simulation results matched the measured heat transfer data well. Optimal width of hydrophobic stripe Wo DWC is found to be dominated by δp and WFWC, but other parameters weakly influence Wo DWC. Interfaced by a δp−WFWC transition curve, a heat transfer regime map is presented to contain Regime I for possible heat transfer enhancement and Regime II for heat transfer deterioration. Regime I enhances heat transfer if WDWC approaches Wo DWC, but may deteriorate heat transfer if WDWC deviates Wo DWC too much. The maximum heat transfer enhancement ratio is 1.67 compared with purely hydrophobic surface. Regime II always deteriorates heat transfer. Our work provides a general guideline to design biphilic surface for performance improvement.

Introduction

Condensation heat transfer enhancement is important to reduce equipment size, metallic consumption and investment cost of condensers. Condensation includes filmwise condensation (FWC) on hydrophilic surface and dropwise condensation (DWC) on hydrophobic surface. The liquid film thickness plays important role on FWC [1]. Various methods have been proposed to decrease liquid film thickness to enhance FWC [2–4]. Heat transfer coefficient of DWC is one or two magnitudes higher than that of FWC [5]. For DWC, the droplet detachment radius rmax greatly affects heat transfer [6], noting that rmax is also the maximum radius of droplet that can stay on hydrophobic surface. When a droplet reaches rmax, the droplet can be detached. DWC can be enhanced by reducing rmax. The value of rmax depends on droplet detachment modes. Xie et al. [7,8] investigated droplet detachment modes on uniform wettability surfaces, including sliding, rolling and jumping. For contact angle θ<126ο, droplet detaches the surface by sliding mode. For θ>147ο, droplet detaches the surface by rolling mode. Besides, the droplets-coalescence-induced-jumping can occur on nanostructured superhydrophobic surface, decreasing rmax [9]. Such surface is expected to enhance DWC. Many studies have been reported about DWC on superhydrophobic surface.

1- Introduction

2- Mathematical model and calculation conditions

3- Gas–solid heat transfer characteristics of packed binary-size particles

4- Gas–solid heat transfer characteristics of packed quintuple-size particles

5- Comparative study of the heat transfer characteristics of packed multi-size particles with the same mean diameter

6- Conclusions

References

Abstract

Real industrial particles generally have a wide size distribution. Therefore, the gas–solid heat transfer characteristics of packed multi-size particles should be studied. A mathematical model of gas–solid heat transfer for packed multi-size particles is established. This model includes gas–solid convection heat transfer and intraparticle and interparticle conduction. The cooling processes of packed binary- and quintuple-size particles ranging from 10 mm to 60 mm under different conditions are investigated. The EDEM software is used to obtain the porosities of different cases. Results show that the presence of small particles in the packed multi-size particles reduces porosity and increases specific surface area, thereby benefiting the gas–particle heat transfer process. The temperature of large particles is always higher than that of small particles during particle cooling. Particle–particle conduction helps in the cooling process of large particles, and the maximum heat flux ratio of interparticle conduction to gas–solid convection for large particles reaches 0.196. The volumetric heat transfer coefficient of the packed multi-size particles varies with time. The initial heat transfer coefficient is the average value weighted by mass fractions, and the limit of the final value is that of the large particle under the actual porosity. The proposed dimensionless volumetric heat transfer coefficient can be a general description of gas–solid heat transfer characteristic of various packed multi-size particles. Its time variation can be well described by an exponential correlation, and the variation rate is related to the variance of particle size in each case.

Introduction

Fixed and moving beds with particles are widely used in industrial applications, such as waste heat recovery [1], gas separation [2], and chemical looping combustion [3]. The gas–solid heat transfer that occurs between the flowing gas and packed particles plays a vital role in determining the performance of such devices [4]. Numerous studies have been conducted to investigate the gas– solid heat transfer characteristics of various packed particles in the last decades [5,6], and information regarding packed mono-size spherical particles has been successfully achieved. Ranz and Marshall [7] theoretically studied the heat transfer process of gas flows over a single sphere and derived a relation for predicting the heat transfer intensity of a single sphere in Nusselt form depending on the Prandtl and Reynolds numbers. Wakao et al. [8] improved an expression on the basis of the experimental data of packed sphere beds. Their result had the same form as but different coefficient from the result of Ranz’s study, which excluded the porosity. Kunii and Levenspiel [9] proposed a heat transfer expression in an improved form, including the porosity of the packed beds. Similar studies were conducted by Gnielinski [10] and Achenbach [11], and corresponding correlations were established to extend the range of the Reynolds number, porosities, and Prandtl number. These expressions perform well in predicting the gas–solid convective heat transfer of packed mono-size spherical particles under various conditions. However, the effects of heat conduction inside a sphere should be considered when the particle size is large. Jeffreson [12] proposed a modified effective heat transfer coefficient for gases and spheres by incorporating the effects of intraparticle conduction, which is based on a theoretical solution of the heat conduction process inside a sphere. Furthermore, Kye et al. [13] derived the expression of a gas–solid volumetric heat transfer coefficient on the basis of the representative elementary volume method of porous medium.

Introduction

Mathematical modeling of the interaction between the hydride tank and the fuel cell

Experimental validation of the hydride tank model

Simulation of the hydride tank-fuel cell interaction

Conclusion

References

Abstract

This paper presents a zero-dimensional (0D) model of hydride tank. The model aims to study the dynamic heat and mass transfers during desorption process in order to investigate the thermal-fluidic behaviors of this hydride tank. This proposed model has been validated experimentally thanks to a tailor-made developed test bench. This test bench allows the hydride characterization at tank scale and also the energetic characterization. The simulation results of the heat exchanges and mass transfer in and between the coupled reaction bed, show good agreement with the experimental ones. It is shown that the heat produced by a Proton Exchange Membrane Fuel Cell (PEMFC) (estimated starting from an electrical model) is enough to heat the metal alloy (FeTi) and therefore release the hydrogen with a sufficient mass flow rate to supply the PEMFC. Furthermore, the obtained results highlight the importance of the developed model for energy management of the coupling of fuel cell and hydride tank system.

Introduction

Fossil fuel reserves depletion and the adverse effects of climate change have attracted global attention and pose serious threats to mankind. The development of new energy technologies based on new materials is vital if the world is to arrest the adverse effects of climate change and secure the global energy security based on sustainable and renewable energy sources. Among the various possible solutions, hydrogen appears as a very attractive energy carrier to progressively establish itself in the economy that is today based on fossil fuels. Hydrogen has a very high energy content of 243 ½kJ=mol, which makes it a very attractive, lightweight energy carrier. Aside from being the most abundant element in the universe, hydrogen is lightweight, and can be produced from all primary resources such as fossil fuels, natural gas and coal, as well as renewable resources, such as biomass and water with input from renewable energy sources (solar, wind, wave or hydro-power …). Because hydrogen is not available anywhere as a separate element, it needs to be separated from the above mentioned sources, for which energy is necessary to do this disassociation.

1- Introduction

2- Experimental procedure

3- Results and discussion

4- Conclusion

References

Abstract

Nowadays, solar energy is harvested in two different ways including the extraction of thermal energy in solar collectors and electrical energy generation in photovoltaic panels. The Photovoltaic panels convert a small fraction of absorbed solar radiation into electrical energy and waste the rest in the form of thermal energy that results in increasing the panel temperature and decreasing the electrical efficiency. Photovoltaic thermal systems (PVT) equipped with phase-change materials (PCM) are capable of benefiting from the storage when phase change happens. In this manuscript, the effect of PCMs deployment on the performance of an air-cooled photovoltaic system is investigated, experimentally. As such, the effect of PCM is deliberated in a setup provided in which the PVT is equipped with a sheet of PCM. Herein, the first case considers a natural convection and the other three cases regard three different forced air convection. The experimental results indicate that using PCM sheets of six millimeters thick leads to reducing the panel temperature to 4.3, 3.4, 3.6 and 3.7 °C in average in a natural flow mode, forced high-velocity, medium and low velocity, respectively. Moreover, decreasing the temperature results in increasing the outlet power and electrical efficiency. Accordingly, it is concluded that using PCMs leads to a significant increase in natural and forced convection situations.

Introduction

The building precinct consumes more than 39% of the total consumed energies in the world and due to using conventional energy resources, it is responsible for a major part of environmental threats including pollution propagation and global warming [1]. To solve the problem, considerable attention has been given to the renewable energy resources. Solar energy harvesting by photovoltaic (PV) technology which can convert solar radiation into electric energy by photovoltaic effect is the most popular one [2]. One of the challenges in the utilization of PV technology is that all of the incident solar radiation to the PV cell surface do not convert to electricity. The maximum efficiency is in the range of 5–20%, and the rest is dissipated as heat raises the PV temperature [3]. Increasing PV cell temperature leads to further decrease in the PV efficiency which worsens the situation. Therefore, cooling technologies are essential to control the rise in temperature and to enhance the performance of the solar cells [4]. It is demonstrated that each 1 °C increase in temperature of a typical silicon-based PV panel leads to decrease in efficiency of around 0.5% [5]. Another study revealed that at 1000 Wm−2 solar radiation power, the produced electricity would be decreased from 240 W to 195 W when the surface temperature rises from 0 °C to 75 °C [6]. Moreover, it is reported that in the crystalline silicon panel the power of module will decrease approximately 0.4–0.65% per each 1 K increase of temperature, if the temperature rises 25 °C [7,8]. To address the issue, many types of research have been accomplished on the cooling of PV panels using PVTs [9]. Specifically, a simple way is to cool the panel by heat transfer fluids such as water or air [10,11]. In PVT systems, the objective is to extract the extra heat from the PV surface by using an operating fluid. The hot fluid can be used for endothermic processes in industrial or residential applications. The fluid can flow through the system in natural or forced circulation ways [12]. In Ref. [13], it is shown that the forced ventilation provides higher heat transfer than the natural flow circulation. However, it is necessary to use pumps (for water) or fans (for air) to generate forced convection. Natural or forced air circulation is a simple and low-cost way to remove heat from PV panels while water heat extraction is more expensive and causes pressure stresses and electrical problems for the PV cells due to its exposure to the water flow [12].

1- Introduction

2- Materials and methods

3- Results and discussions

4- Conclusions

References

Abstract

Advanced heat removal technologies are critical for high-performance automotive engines. The conventional fluids being used today are based on a mixture of distilled water (DW) and ethylene glycol (EG), which widens the operational temperature range but at the same time limits the heat removal. Therefore, the use of nanofluids for improving heat transfer performance has soared over the past few years. The problem is that most of the reports highlight the short-term heat transfer results which may not be true over time. In this paper, a suggested best practice for analyzing the usage of nanofluids in heat transfer applications is presented, specifically for an actual car radiator. This work investigates the use of aluminum oxide (Al2O3) and titanium dioxide (TiO2) nanoparticles dispersed in DW and EG at 50:50 volumetric proportions. The choice of these oxide-based nanofluids is motivated by their anti-corrosive properties that are usually not analyzed or discussed in most of the articles. Furthermore, the emphasis is given on the presentation of a comprehensive characterization of the nanofluids including thermophysical properties (size, density, viscosity, thermal conductivity, corrosive behavior) and long-term stability (zeta potential) which are essential for an end-user to have. The results showed a maximum enhancement of the thermal performance by 24.21% using Al2O3 at a volume fraction of 0.3%. Friction factor and performance evaluation criterion (PEC) for the radiator experiments are calculated in order to determine the penalty in the pressure drop and to evaluate it properly. Finally, it is found that the values of PEC lie in the range of 1.03–1.31 which indicates significant flow enhancement.

Introduction

The improvement in automotive technologies has led to increased thermal loads, and therefore higher cooling rates are required [1]. The addition of fins, microchannel, and turbulators are the traditional approaches used to increase cooling rates of the radiator which are already extended to their limits [1,2]. In recent years, the automotive industry has been focused on the weight reduction of the vehicle as it improves the fuel economy and the associated running costs [3]. Weight reduction can be achieved in the engine cooling system. Furthermore, many conventional coolants (lubricants and water) have characteristically shown poor heat transfer properties due to their lower thermal conductivities (TCs) [1,4]. Engine performance, fuel efficiency, and emissions are specific parameters of an automobile which are profoundly affected by convective heat transfer [5,6]. Therefore, a new compact and innovative coolants and/or coolant systems are needed to fulfill the growing needs for heat removal in a clean and eco-friendly way [7].

1- Introduction

2- Experimental

3- Model development

4- Results and discussion

5- Conclusion

References

Abstract

The effect of viscosity on heat transfer behaviour is analyzed for binary mixtures (ionic liquid + water and nanofluid + water) over the base fluid (water). Viscosities of 1-butyl-3-methylimidazolium bromide with water under ambient pressure are studied at different concentrations (0.1–0.6% w/w) and temperatures (296–336 K). Viscosities of nanofluid (γ-Al2O3/water) are estimated at the same concentration and temperature range and then compared with the ionic liquid solution. Viscosity of aqueous 1-butyl-3-methylimidazolium bromide solution increases with concentration and decreases with temperature. A similar trend is observed for γ-Al2O3/water. In addition, a model is developed with response surface methodology and artificial neural network to predict the viscosity of 1-butyl-3-methylimidazolium bromide with water.

Introduction

Viscosity is a property which describes the internal friction of a moving fluid. It is an important thermophysical property in fluid flow and heat transfer. In many fluid flow problems, viscosity is assumed to be a constant parameter. However, viscosity varies with temperature when the flow is associated with transfer of heat. To accurately model the flow behaviour and estimate heat transfer rates, it is essential to consider the variation of viscosity with temperature [1,2]. When compared with other thermophysical properties viscosity demonstrates a considerable variation. Hence it becomes necessary to incorporate the correction in the fluid flow model based on the behaviour of fluid with respect to temperature-dependent viscosity [3]. The variation in temperature-dependent viscosity plays a major role in laminar heat transfer and friction factor for non-circular conduits or ducts of different geometries [3–5]. It has been shown that the variation in temperaturedependent viscosity in a curved circular tube with water as a working fluid, plays a relatively major role on velocity and thermal profiles, compared to other thermophysical properties like specific heat, thermal conductivity and density [6]. In regards to viscosity variation, it results in a larger local Nusselt number when compared with that obtained from using constant viscosity [3,7]. The use of constant viscosity ignores the effect of decreased flow resistance leading to the generation of the secondary flow of fluid and its impact on convective heat transfer enhancement. Frictional losses in the fluid – the result of the microscopic interaction of molecules between layers – are governed by the viscosity of the fluid.