Higher Heat Transfer Coefficient Research Articles

A comparative study of the impact on combustion and emission characteristics of nanoparticle‐based fuel additives in the internal combustion engine

AbstractThe search for an effective solution to improve performance and emission characteristics of internal combustion (IC) engines used in the commercial sector is regarded as one of the most important and essential issues in recent years due to increasing levels of pollution. Nanoparticles with their additive ability to increase fuel reactivity and atomization, due to their large surface area and high heat transfer coefficient, can improve the performance and emission characteristics of a fuel. This review highlights the use of nanoparticles as fuel additives to enhance the emission and performance characteristics of IC engines. Detailed comparisons of performance, emission, and combustion characteristics of IC engines using fuels blended with nanoparticles have been done. Nanoparticles were observed to be an oxygen buffer for fuel combustion and boost fuel atomization, thus enhancing engine performance. While alumina exhibited a decrease in levels of HC and CO but a considerable increase in NOx, graphene nanoparticles and ceria were found to be particularly effective in enhancing engine performance. Detailed study has been done on other nanoparticles, including metal‐oxide, nonmetal‐oxide as well as carbon nanoparticles. Overall, the use of nanoparticles can enhance the thermophysical characteristics of fuels, improving the emission and performance characteristics of engines. The review suggests that selecting the right dosage and variety of nanoparticles is crucial for optimizing engine performance, and thus directly helps in tackling the ongoing pollution problem.

Experimental investigation of the convective heat transfer coefficients at different angular orientations inside the void in a cold flow pebble bed reactor using a non-invasive heat transfer pebble probe

This study focuses on investigating the effect of the surface location of the pebble inside the bed, by varying the angular orientation of the heat transfer pebble probe and the velocity of the gas on the convective heat transfer coefficients in a cold-flow pebble bed reactor. An advanced non-invasive technique consisting of a cartridge heated copper pebble, a micro-foil sensor and a thermocouple probe was used to obtain accurate measurements of the local heat transfer coefficients from three surface locations inside the void. The effect of the wall on the local convective heat transfer coefficients was highlighted due to the low aspect ratio of the bed (bed diameter to pebble diameter (5cm)) used in this work. The local measurements showed, for the first time ever, that the surface location inside the void substantially impacts the heat transfer coefficients, with higher heat transfer coefficients at the center of the void and bottom of the void. Furthermore, the measured overall heat transfer coefficients showed good agreements with the predictions of the correlations of [52,21] and [14]. A second order polynomial model with an R2=0.9807 and an average absolute relative error (AARE) equal to 9.08%, was developed for the prediction of the local heat transfer coefficient within the design and operation conditions of this work. The accurate local heat transfer data collected in this work is valuable as benchmark data for the validation of computational fluid dynamics (CFD) simulations and heat transfer calculations.

Numerical study of falling film flow and heat transfer on the horizontal tube with a porous coating layer

Porous surfaces often exhibit exceptional performance in liquid spreading and heat transfer, making them promising candidates for enhancing falling film heat transfer on solid surfaces. This paper delves into the flow and heat transfer characteristics occurring within falling films on horizontal tubes coated with a porous layer. To achieve this, a three-dimensional numerical model that couples the Volume of Fluid (VOF) method with porous media considerations was employed. The investigation comprehensively covers film spreading behavior, flow patterns, film velocity, film thickness, temperature distribution, and heat transfer coefficients, for both plain and porous tubes. Furthermore, we delve into the effects of key parameters, such as porosity, layer thickness, and material properties, on the overall heat transfer process. The results demonstrate the accuracy of our model in reproducing film thickness and heat transfer coefficients within falling films on horizontal porous tubes. When comparing plain tubes to porous ones, some key differences emerge. Specifically, the liquid film on porous tubes spreads more slowly, leading to increased film thickness and fewer occurrences of dry patches. Additionally, porous tubes result in lower film velocity and a more uniform temperature field compared to plain tubes. Interestingly, the findings reveal that smaller porosities yield thicker films and higher heat transfer coefficients, particularly when the porosity is below 0.4. Moreover, thicker porous layers result in thinner films, higher wetting ratios, and increased heat transfer coefficients. Among the materials we tested, aluminum and copper porous layers exhibit the highest heat transfer performance, while steel demonstrates the lowest.

Pool Boiling of Ethanol on Copper Surfaces with Rectangular Microchannels

In this paper, pool boiling of ethanol at atmospheric pressure was analyzed. The enhanced surfaces were made of copper, on which grooves with a depth ranging from 0.2 to 0.5 mm were milled in parallel. The widths of the microchannels and the distances between them were 0.2 mm, 0.3 mm and 0.4 mm, respectively. The highest heat transfer coefficient, 90.3 kW/m2K, was obtained for the surface with a microchannel depth of 0.5 mm and a width of 0.2 mm. The maximum heat flux was 1035 kW/m2. For the analyzed surfaces, the maximum heat flux increase of two and a half times was obtained, while the heat transfer coefficient increased three-fold in relation to the smooth surface. In the given range of heat flux 21.2–1035 kW/m2, the impact of geometric parameters on the heat transfer process was presented. The diameters of the departing bubbles were determined experimentally with the use of a high-speed camera. A simplified model was proposed to determine the diameter of the departure bubble for the studied surfaces.

Optimized Design of Shell and Tube Heat Exchanger with Segmental Baffles

Heat exchangers are the apparatus which is widely used in various industries. They transmit heat among two or more fluid streams. Theoretical analysis done by Kern’s method has been done on Shell and tube heat exchanger with segmental baffle and then experimented in CFD to check Heat Transfer rate and pressure drop with varying number of baffles which is 6, 8 and 10, and by keeping all the rest of parameters constant. It is found that as the number of baffles increase heat transfer rate increases also pressure drop is increases significantly. To optimize model in next step, CFD analysis is has been done by varying baffle cut to 25%, 35% and 45% by keeping same number of baffles which found best in heat transfer rate in earlier Kern’s method. Other parameters like velocity, temperature, pressure and baffle numbers are kept constant. Results obtained from numerical solution are analysed extensively to get the effect of baffle cut on heat transfer rate and pressure drop on shell side. The pressing need of the heat exchanger industry, the tradeoff optimization between the pressure drop and the heat transfer coefficient has been studied to provide an idea to get effects of the change in two parameters, namely, baffle spacing and baffle cut simultaneously and observe how these two will affect the performance of an STHX. An attempt to find the optimum geometric configuration has been carried out with number of baffles. Increasing baffles for same length of shell and baffle cut increases from 25% to 35%, heat transfer rate increases. It is beneficial in elucidating that a higher heat transfer coefficient can be obtained for fewer baffles when coupled with the appropriate baffle cut. So selection of correct optimized shell and tube heat exchanger. The work is carried out for copper tube bundles and steel shell. So optimization of baffle cut with correct number of baffles and reduction in pressure drop studied with CFD tool.

Experimental study of R448A flow boiling in a 24-port microchannel tube

R448A is a mixture to replace R404A in the thermodynamic cycle applications. R448A has a temperature glide of about 5°C and might exhibit thermal non-equilibrium in the microchannel tube. This study investigated the flow boiling heat transfer and pressure drop of R448A, a five-component non-azeotropic mixture, in a microchannel tube. Six heat transfer coefficients (HTC) and pressure gradients (dP/dz) of R448A were measured in one pass in a 24-port microchannel tube. The experimental conditions covered mass fluxes from 100 to 200 kg-m−2s−1, heat fluxes from 2 to 8 kW- m−2, and saturation temperatures from 10 to 30 °C. The study compared the experimental results of R448A with four of its component fluids (R134a, R1234yf, R1234ze(E), and R32). R448A had higher HTC and lower dP/dz than its component fluids, except for R32 which had higher HTC and dP/dz than R448A. Muller-Steinhagen and Heck was the best correlation for predicting the dP/dz of R448A, with a mean absolute error (MAE) of 8.78 % and a mean error (ME) of 7.47 %. A new correlation for predicting the HTC of R448A, based on modifying Liu and Winterton (Müller-Steinhagen and Heck, 1986), was proposed with an MAE of 6.01 % and an ME of 0.588 %.

Response surface methodology-based multi-nozzle optimization for electrospray cooling

The performance of electronic systems has decreased due to an increase in heat flux density. New methods such as electrospray cooling are promising to ensure that these systems operate under desired conditions. In electrospray cooling, the working flow rate is limited because the small nozzle diameter is preferred to improve the electric field strength and heat transfer performance. However, it is known that increasing the fluid flow rate is a good method to improve heat transfer. For this reason, multi-nozzles should be used to remove more heat from the surface with electrospray cooling. For the first time in the literature, multi-nozzles were optimized for electrospray cooling with the optimization method in the study conducted for this purpose. Nozzle optimization studies were carried out for ethanol–water mixture used as a fluid for the first time in the literature, nozzle diameter, distance between nozzles, and nozzles number. In the experimental study, it was determined that the fluid composition ratio was effective on electrospray cooling and the effects of nozzle geometry parameters (nozzle diameter, distance between nozzles, nozzle number) were limited. As a result of the optimization process based on the response surface methodology, the highest heat transfer coefficient was obtained for the 5 mm nozzle-center distance where 3 pieces of 26G nozzles.

Effects of shark bionic V-groove structures and nanofluids on thermal management of electronic components

The miniaturization of electronic components makes it difficult for the traditional radiator working medium and radiator structure to meet the requirements. The combination of nanofluids and bionic micro-surfaces is beneficial to solve the heat dissipation problem of electronic components. Based on the bionics, a CPU enhanced heat dissipation system was designed and built based on the V-shaped groove of shark scales and Fe3O4-H2O nanofluids with high heat transfer coefficient. The effects of V-groove angle, height and nanofluids concentration on the comprehensive evaluation index of the radiator were studied. The groove angle S= 1 mm (54°) and height H= 2 mm have the best heat transfer characteristics and comprehensive evaluation index, which can reduce the surface temperature by 2.16 °C and 3.71 °C at most, and increase the heat transfer efficiency by 17.58% and 28.96%. There is a critical concentration (0.3%) that makes the system cooling effect best. The results of the study have a certain reference value for the structural design of heat sink

Review and Analysis of Heat Transfer in Spacer-Filled Channels of Membrane Distillation Systems.

Membrane distillation (MD) is an attractive process for the concentration of seawater brines. Modelling and simulation of membrane distillation processes requires a better knowledge of the heat transfer coefficients in spacer-filled channels which are usually determined by applying empirical correlations for the Nusselt number. In this study, first, a comprehensive literature review on heat transfer correlations was conducted. It was found that the empirical correlations often used for MD simulation result in strongly varying Nusselt numbers that differ by up to an order of magnitude at low Reynolds numbers. Then, heat transfer in spacer-filled channels was investigated experimentally in a membrane distillation system using an aluminum plate instead of a flat-sheet membrane. Numerous tests were carried out with sodium chloride solutions in a wide range of salinities, between 1 g/kg and 95 g/kg, and temperatures, between 30 °C and 80 °C, yielding high heat transfer coefficients in a range of 1500 to 8300 W/(m2K) at relatively low Reynolds numbers, between 100 and 1500, clearly showing the influence of the spacers on heat transfer. A new empirical Nusselt correlation (Nu=0.158Re0.652Pr0.277) was derived which represents the experimental data with a deviation of 10% and is valid for 100<Re<1500 and 2<Pr<7. Computational fluid dynamics simulations were performed to analyze the variations of the fluid properties across the boundary layer due to temperature differences. The simulations showed only minor deviations of the heat transfer coefficients in the hot and cold fluid channels for small driving temperature differences.

Water, ethanol, or methanol for adsorption chillers? Model-based performance prediction from Infrared-Large-Temperature-Jump experiments

Adsorption chillers are a promising technology to ease the burden on renewable electricity demand by using waste heat as driving energy. Their performance depends on the adsorbent’s kinetics, the chosen refrigerant, the design of the adsorber, and the selected temperatures. Thus, quantifying an adsorption chillers’ performance usually requires expensive experiments at full scale. Here, we efficiently characterize a broad set of working pairs for a wide range of conditions using a two-step approach: First, we conducted Large-Temperature-Jump and equilibrium experiments to extract kinetic and equilibrium parameters of water, ethanol, and methanol with a set of currently discussed adsorbents. Second, we inserted the parameters into a dynamic Modelica model to evaluate the working pairs’ performance in full-scale two-bed adsorption chillers. For chilling above 0 °C, water remains the benchmark refrigerant with the highest volumetric power and efficiency with the silica gels SG123 and Siogel. Ethanol and methanol working pairs offer higher mass transfer coefficients, but could not offset the benefits of water. Coatings of the MOFs aluminum fumarate and CAU–10–H provided exceptionally high heat transfer coefficients, but were too thin to compete. For chilling below 0 °C, ethanol and methanol were on par. ZIF–8 was competitive with the activated carbon CarboTech A35. Overall, CarboTech A35/methanol showed broad applicability, performing comparably to SG123/water above 0 °C while still providing reasonable performance below 0 °C.

Numerical investigation on necessary condition for temperature oscillation in loop heat pipe

A loop heat pipe (LHP) is a two-phase heat transfer device that utilizes the latent heat and capillary force of a wick. Many spacecraft have been equipped with LHPs because of the ability of LHPs to transfer heat efficiently. However, temperature oscillations may sometimes occur and cause deviations from the allowable temperature range. A recent numerical analysis of temperature oscillations has revealed some sufficient conditions for temperature oscillations. However, the necessary conditions, which are required to prevent temperature oscillations, are not yet understood. Here, a parametric study was performed using a transient mathematical model to understand the necessary conditions for temperature oscillations. The parametric study showed that a small heat capacity of the reservoir, low-thermal-conductivity wick, or high evaporation heat transfer coefficient led to temperature oscillations. These results indicated that temperature oscillations occur when the temperature difference between the reservoir and condenser becomes small. Therefore, a small temperature difference between the reservoir and condenser is a necessary condition for temperature oscillations. This is because the small temperature difference causes the penetration of the two-phase flow into the liquid line. To prevent temperature oscillations, a large temperature difference should be maintained. One method to maintain the large temperature difference is to increase the thermal capacity of the reservoir, such as by increasing the filling ratio. Heating the reservoir is another method to keep the temperature difference high.

Numerical simulation and optimization of heat pipes using nanoparticles: a feasibility study

This study investigated the effect of different particle sizes on the thermal effectiveness of a cylindrical screen mesh heat pipe using silver nanoparticles as the test substance. The study compared the effects of three different particle sizes (30, 50, and 80 nm) on heat transmission characteristics of the heat pipe. The results showed that the heat pipe’s thermal conductivity increases as the particle size decreases. The thermal conductivity measurements revealed 250 W/mK, 200 W/mK, and 150 W/mK for particles of 30, 50, and 80 nm sizes. Furthermore, the study analyzed thermal resistance of heat pipe for each particle size and found that as particle size decreases, thermal resistance decreases as well. The heat transfer coefficient values for particle sizes of 30, 50, and 80 nm were found to be 5500 W/m2K, 4500 W/m2K, and 3500 W/m2K, respectively. The ANOVA method was used to analyze the results statistically. The 30 nm particle size was found to have significantly higher thermal conductivity, lower thermal resistance, and a higher heat transfer coefficient than other particle sizes. Additionally, simulations were conducted to analyze the temperature variation of the hot source as a function of power dissipation for design and optimization.

Coupling dynamic thermal analysis and surface modification to enhance heat dissipation of R410A spray cooling for high-power electronics

With high critical heat flux (CHF) and heat transfer coefficient (HTC), spray cooling is considered as one of the most promising thermal management technologies for high-power electronic devices. To increase its cooling performance, a closed-loop experimental rig was constructed to study the effects of spray and system parameters on heat transfer enhancement by R410A. The best cooling performance can be achieved under optimal subcooling degree of 17 °C and nozzle diameter of 0.56 mm. When the compressor frequency reaches the upper limit of 90 Hz, maximum CHF and HTC on flat surface are 301.6 W/cm2 and 91.7 kW/(m2·K). To further improve CHF, mechanism of heat transfer enhancement by square pin finned surface was revealed in terms of droplet splashing. With fin width of 0.5 mm and height of 3 mm, CHF as high as 522.1 W/cm2 and peak HTC of 407.0 kW/(m2·K) are reached, while maintaining the cooling surface temperature lower than 55.6 °C. Compared to flat surface, CHF and HTC are enhanced by around 73.4% and 3.5 times, respectively. Based on the experimental data, CHF correlation applicable to pin finned surface was obtained with precision of ±12.4% by introducing fin height and width.

Experimental investigation of heat transfer performance in gas-atomized spray cooling

Spray cooling has substantial potential for wide application, given its unique characteristics, including high heat transfer coefficient, absence of contact thermal resistance, and the elimination of cooling hysteresis. In this study, a visualized experimental system featuring a gas-liquid two-phase cyclonic nozzle was constructed to study both the heat transfer and fluid morphology features pertaining to the wall. With the increase of wall superheat, a progressive transition across four distinct stages of heat transfer – liquid film, stream flow, droplet flow, and mist flow can be found. Of these, the droplet flow stage outperforms others in terms of heat transfer performance and is thus advocated as the preferred mode of application. In response to increasing spray pressure, the droplets generated on the wall during the droplet flow stage undergo a reduction in size, yet become more densely distributed. A notable increase in critical heat flux by 59.5 % is observed when spray pressure is elevated from 0.2 to 0.4 MPa. While raising the spray flow rate leaves the heat transfer performance during the liquid film stage largely unchanged, it effects a significant augmentation in critical heat flux. Our experimental results ultimately obtain the optimal operating parameters that facilitate the wall surface's retention in the droplet flow stage. These experimental results are poised to contribute meaningfully to the engineering practices associated with spray cooling applications.

Numerical simulation of flow characteristics of falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube

Horizontal-tube falling-film evaporator is regarded as an effective alternative to full-liquid evaporator because of its higher heat transfer coefficient and lower refrigerant charge. In the process of falling-film evaporation, the thin liquid film thickness (δ) is an important parameter for characterizing the hydrodynamics, and accurate prediction of δ can effectively prevent the reduction in heat transfer efficiency caused by local wall dryout. In this study, non-azeotropic refrigerant R32/R134a was taken as the research object. By incorporating the multi-component phase change model and contact angle model into the governing equations, a two-dimensional numerical model of falling-film evaporation outside a horizontal circular tube was established. The effects of the spray height (H), tube diameter (d), Reynolds number (Re), inlet temperature (Tinlet), and mass fraction of R32 in the liquid-phase mixture (MR32_liquid) on δ were analyzed under sheet flow. The results show that an increase in Tinlet, H, and d is favorable for increasing the average δ (δave), whereas an increase in MR32_liquid and Re leads to a reduction in δave. Besides, the decreasing rate of the δave decreases gradually as H and d increase. When H is 4 mm, the local δ (δlocal) first decreases and then increases as the circumferential angle (Φ) increases. When H ranges from 6 mm to 13 mm, as Φ increases, δlocal first increases slightly, then decreases, and finally increases significantly near the wake region. The δlocal of a larger tube diameter is thicker than that of a smaller tube diameter near the impact region, whereas in the subsequent regions, the δlocal of a larger tube diameter is thinner than that of a smaller tube diameter. The minimum δlocal occurs at Φ between 130° and 140°, and with an increase in H and d, Φ corresponding to the minimum δlocal decreases.

Hierarchical hybrid heat sink material for thermo-electric generators

This short communication focuses on the impact of enhancing heat sink effectiveness of thermoelectric generators (TEG) by attaching covalently bonded carpet-like arrays of carbon nanotubes on prefabricated copper structures. Structural hierarchy of heat sinks are important for synergizing the thermal conductivity of materials with fin architectures tailored for active or passive electronic cooling components. We investigated strategic integration of carefully aligned carpets of carbon nanotubes (CNTs) on heat sink fins to simultaneously enhance conductive, convective, and radiative heat transfer pathways. Carbon nanotubes (CNT) combine ultra-high thermal conductivity along their axial directions with high surface area and radiative heat transfer coefficients. While CNTs are known to enhance nanofluidic coolants and thermal interface composites, their full potential can only be utilized if they are aligned normal to heat sink fins, and strongly anchored to retain the fin/radiator architecture in service conditions. This study focused on measuring the direct impact of CNT carpet on TEG performance under ambient passive cooling conditions without adding any noticeable weight or volume to the heat sink. It was seen that this single step of CNT attachment on the surface could increase the open circuit voltage of TEG modules by over 50% in the passive mode with respect to original untreated heat sink, and over 100% with respect to no heat sink. This improvement may be enhanced even further by introducing forced coolants. These results therefore show significant promise in using the surface nanocarpet approach for development of compact, lightweight, robust and reusable solid-state heat sinks in future.

Hydrodynamics of a Multisized Particle Mixture in Shallow Fluidized Bed With Immersed Tubes

Abstract Inert solid particles have made the integration of more efficient power cycles with super high operating temperatures into the concentrated solar power (CSP) station feasible. Fluidized bed heat exchangers with feature of high heat transfer coefficient have great application potential in the particle-based CSP. However, the parasitic loads of the additional fluidizing gas loop and the finely sieved monosized particles may deteriorate the economic efficiency of the integrated system. In order to cope with this problem, a conceptual design of a shallow fluidized bed (SFB) heat exchanger is proposed for the new generation CSP technology. Fluidization characteristics of bauxite particles with a nonuniform particle size distribution in SFB with immersed tubes are investigated with a combination of experimental measurements and computational fluid dynamics simulations. Results show that the static bed height and opening area ratio of the distributor has insignificant influence on the range of semifluidized region and the minimum fluidization velocity Umf. The standard deviation of bed pressure drop σ in the grid region can be used as an alternative criterion for identifying the fluidization state. A range of superficial velocity that distinguishes two different solid circulation patterns exists, with its boundary values being four times and eight times the Umf, respectively. The immersed tubes can inhibit the asymmetric particle circulation patterns from developing in the SFB, but cause a substantial increase in the σ within the grid region.

Numerical and experimental analysis of air-cooled Lithium-ion battery pack for the evaluation of the thermal performance enhancement

The main objective of this study is to assess the thermal performance of an air-cooled Lithium-ion battery pack. This involves analyzing the heat dissipation characteristics and temperature distribution within the battery pack at different operating conditions. Here, the thermal performance of the battery pack is evaluated numerically and experimentally. The experimental platform allows adjusting numerous control factors, like air velocity (0.5 to 2.0 m/s), air temperature (282.0 to 315.0 K), and battery pack discharge rate (0.5 to 2.5C). The temperature distribution is studied in longitudinal and transverse directions of the battery pack and circumferentially for a cell. The numerical simulations involve the solution of Shear Stress Transport (SST) k- ω model-based Navier-Stokes (N-S) equation using the ANSYS FLUENT R19.2 software. The numerical results were found to closely match the experimental findings with maximum error to be within 3.12%. The evaluation parameters used to assess the thermal performance of BTMS are maximum temperature(Tmax), maximum temperature difference (∆Tmax), average temperature (Tavg.), and standard deviation of the temperature σT within the battery pack. The numerical and experimental analyses show a reduction in both the maximum and mean temperature of the battery pack and improved temperature uniformity, with the increase in air velocity, lowering the discharge rate and decreasing the ambient temperature. The correlation between air inlet velocity and temperature drop is stronger at higher discharge rates than lower discharge rates. The effect is due to high heat transfer coefficient and uniform air distribution at high velocities, less heat generation and more time to dissipate heat at low discharge rates, and lower temperature of air at cold ambient conditions. The reduced temperatures and temperature uniformity minimizes the occurrence of localized hotspots within the battery pack. Further, for any given air inlet temperature, an increase in temperature is observed parallel to the direction of airflow. Under severe operating conditions, the inlet-to-discharge temperature difference reached a maximum of T = 22.14 K whereas, under mild conditions, it dropped to T = 0.21 K. The circumferential temperature non-uniformity of cells lying in high-temperature zone of the battery pack reduces with increasing velocity. The work presented here can help in the identification of potential hotspots and design better cooling strategies for the battery pack at different operating conditions.

Multiscale enhancement of refrigerant falling film boiling by combining commercially enhanced tubes with nanostructures

Multiscale surface structures offer the opportunity to combine the heat transfer enhancement provided by microscale structures with the dryout benefits provided by some nanostructures, which is particularly attractive for falling film evaporators, who have to prevent dryout to ensure high heat transfer coefficients can be maintained.In this study commercially produced plain, low-finned and 3D enhanced commercially produced tubes were tested uncoated as well as coated with fibrous nanostructures that induce wicking on the surface to investigate the opportunity that multiscale enhancements might offer falling film boiling evaporators. Single tubes were heated internally by water and tested in the horizontal position with refrigerant R-134a drizzled over the outside of the tube surface at saturation temperatures of 5 and 25 °C, heat fluxes from 20 to 100 kW/m2 and film Reynolds numbers of up to 2000. The tubes tested were a roughened plain tube, a low-finned Gewa-KS tube and two different 3D enhanced tubes, an Gewa-B5 and an Turbo EHPII, that have networks of re-entrant cavities.The uncoated 3D enhanced tubes achieved the highest heat transfer coefficients, outperforming a roughened plain tube by up to 500 %. The uncoated low-finned tube had heat transfer coefficients that were up to 140 % higher than the roughened plain tube, despite having only an 80 % larger surface area, thought to be due to bubbles sliding down the channels between the fins, increasing the surface area of the bubbles in contact with the surface and thus allowing for increased microlayer evaporation.The application of the copper oxide coating benefitted the low-finned tube, with heat transfer enhancement of up to 60 %, possibly due to liquid wicked underneath the sliding bubbles and onto the fin tips. However, the nanocoating largely decreased the heat transfer of the roughened plain tube and the 3D enhanced tubes, thought to be due flooding of nucleation sites on the roughened tube and interference with the hydraulics of the re-entrant cavity network on the surface of the 3D enhanced surfaces.The low-finned tube also benefitted the most from the falling film boiling heat transfer mode compared to previous pool boiling results, with heat transfer increased by up to 80 and 460 % for the uncoated and coated low-finned tube respectively, again likely due to the bubbles sliding in the fin channels under falling film conditions.The Gewa-B5 tubes had the best dryout performance, followed by the Turbo EHPII tubes and then the low-finned Gewa-KS tube. The open pore structure of the Gewa-B5 likely aided it in dryout prevention, while the fins of the Gewa-KS are thought to have prevented lateral fluid distribution and thus worsen dryout.The dryout performance of the tubes we not markedly improved by the nanocoating, highlighting the difficulty in improving wettability on high wetting low surface tensions fluids such as refrigerants. Microscale enhancements were thus shown to have significant influence on dryout performance, while nanoscale enhancements had little effect in this study.

Investigation of the thermal management potential of phase change material for lithium-ion battery

With the rapid development of electric vehicles, lithium-ion battery has been widely used as an energy source. However, to avoid thermal runaway and to ensure the correct operation of lithium-ion battery system, effective battery thermal management would be required. In this study the capacity of phase change materials with different thermal-physical properties to respectively manage thermal issues associated with lithium-ion battery is theoretically investigated using computational fluid dynamics. The impacts of shell material, heat transfer coefficient, phase change material fill volume and the shape of the battery pack on the thermal performance of the lithium-ion battery/phase change material system are investigated in detail. The results show that when the ambient temperature was 20 °C or 30 °C, phase change material-RT35 shows the best temperature control ability for the proposed system, while the phase change material-RT50 has better thermal management capability when the ambient temperature is 40 °C. Also, higher heat transfer coefficient or shell material with higher thermal conductivity, effectively reduce the maximum temperature of the lithium-ion battery and the temperature difference within the proposed system. Specifically, the phase transition time in the lithium-ion battery/phase change material system with a heat transfer coefficient of 10 W/m2K is more than twice longer compared to that of 1 W/m2K and the maximum temperature is 47.18 °C after 150 min of charging and discharging. It is also observed that the phase change material with larger volume ratio leads to lower system temperature during phase transition and the continuous charging and discharging after completing transformation of the phase change material impacts negatively on the thermal management of the lithium-ion battery system. Besides, the rectangular lithium-ion battery/phase change material system surpasses the circular one by 14.78 °C in maximum temperature, while attaining a minimum temperature difference of 9.6 °C after 150 min of charging and discharging.