With the increasing power density of electronic chips, large radar, laser diode array and other equipments, the conventional heat dissipation methods are difficult to achieve the desired thermal ...control requirements increasingly. Spray cooling has attracted widespread attention due to its advantages in high heat flux removal such as less flow rate demand, high heat dissipation capacity, low superheat degree, no temperature overshoot and no contact thermal resistance with the heating surface. As of today, lots of researchers engage in this field and numerous achievements of spray cooling are obtained theoretically and experimentally. In this paper, an overview with spray cooling was completed. The current research progresses of heat transfer mechanisms of spray cooling in the three stages (single-phase regime, two-phase regime and critical heat flux regime) were summarized, and the influence factors, spray characteristics, heating surface characteristics, fluid characteristics and external environment characteristics, were analyzed in detail. The flash evaporation cooling, a special form of spray cooling, was also explored by a number of studies due to its irreplaceable advantage in low pressure environment or in space. Film flash evaporation and droplet flash evaporation significantly improve the cooling capacity of system and utilization of working fluid. In fact, the application of flash evaporation cooling is profound for development and expansion of spray cooling. Additionally, spray cooling system and nozzle were also elaborated in the paper.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•The effect of wall roughness on S-CO2 heat transfer was studied experimentally.•Rough walls can alleviate heat transfer deterioration and enhance heat transfer.•The effect of operation parameters on ...the heat transfer of S-CO2 was discussed.•A correlation was proposed based on experimental data, including roughness effects.
The heat transfer characteristics of supercritical carbon dioxide(S-CO2) are vital for S-CO2 power cycles. Previous studies primarily focused on the heat transfer of S-CO2 in smooth or ribbed tubes. Rough walls can affect the heat transfer in turbulent flows. Whereas roughness effects on S-CO2 heat transfer have rarely been studied. The heat transfer characteristics of S-CO2 in tubes (d=4.57 mm,16207≤Re≤91735) with various roughness (1.5 μm∼40 μm) are investigated experimentally in this study. The results reveal that rough wall helps alleviate heat transfer deterioration, which is embodied in: the critical heat load(q/G) for heat transfer deterioration increases from 0.11 kJ/kg to 0.17 kJ/kg as roughness increases. Heat transfer deteriorates in all tubes under high q/G, the temperature peak and deterioration length drop from 106.9 °C to 73.7 °C and 0.75 m to 0.25 m as roughness increases. No heat transfer deterioration in various tubes under low q/G, the heat transfer coefficient practically triples with increasing roughness. The effect of operation parameters on heat transfer performance of S-CO2 in various tubes is similar. A heat transfer correlation for rough tubes is proposed based on 5460 experimental data, which catches 90% of experimental data within ±20% error.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The effect of vibration on S-CO2 heat transfer was studied experimentally.•Heat transfer deterioration can be mitigated and improved by vibration.•Discuss the effect of vibration on heat transfer ...under various conditions.•The correlation is proposed based on the experimental data.
It is inevitable for mechanical motion to generate vibration, which may affect fluid heat transfer. Although some developments on heat transfer in supercritical carbon dioxide (S-CO2) have been established, there is none research on the impact of vibration on S-CO2 heat transfer currently. The effect of transverse vibration on heat transfer characteristics of S-CO2 in 1200 mm tube is investigated experimentally in this study. The results demonstrate that the vibration enhances the heat transfer of S-CO2 clearly. With the increase of the vibration amplitude, frequency, mass flux, or pressure, the heat transfer enhancement efficiency (HTE) tended to rise, and the HTE of the upper vertex is better than the lower vertex. Within the test range, the highest average HTE is 10.2%. Along the tube, the local HTE in the pseudo critical region is much more noteworthy than in other regions. Finally, the heat transfer correlation is proposed in accordance with the experimental findings, and 95.6% of the data error is within ±15%.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Thermal sensitive flexible PCMs broaden the use of energy storage technology.•Flexible PCMs present thermal sensitive flexibility with Tparaffin,m as stimulus.•Transformation from rigid to ...flexibility is reversible for flexible PCMs.•Thermal contact resistance and poor installation are overcame by using flexible PCMs.•Thermal conductivity of flexible PCMs was improved to a breakthrough level.
Application of thermal energy storage technology in practical engineering has been inhibited due to the constant strong rigidity of phase change materials (PCMs) at any given temperature. To solve this problem, a novel kind of thermal sensitive flexible PCMs was developed by using difunctional olefin blockcopolymer (OBC) replacing conventional supporting materials. Due to phase separation morphology, OBC presents physical crosslinked network and elasticity which is essential for macroscopical elastic deformation of composite. The developed PCMs based on OBC not only exhibit the same high latent capacity and stable shape as shape-stabilized PCMs, but also present thermal sensitive flexibility with the melting point of paraffin Tparaffin,m as the stimulus. Below the melting point of OBC-TOBC,m, the transformation from rigid to flexibility is reversible. The above excellent flexibility of the developed PCMs contributes to reducing thermal contact resistance and improving poor installation for conventional PCMs in energy storage field, such as in energy-storage air-conditioning and compact electron device thermal control, especially in spacecraft thermal control system. Furthermore, thermal conductivity was improved to a breakthrough level (479% of original rate) by adding 3wt.% of the maximum size of expanded graphite, which is based on the guidance of the novel calculation model on account of thermal conductivity for thermal sensitive flexible PCMs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK, ZRSKP
•A new energy electric vehicle thermal management system using CO2 as the medium has been applied in extreme low temperature weather (−20 ∼ -50℃).•Using a coolant system to complete heat distribution ...and self-enhanced enthalpy.•Switching modes to achieve higher efficiency by recovering residual heat changes.•Using self-enhanced enthalpy and waste heat recovery methods to avoid heat absorption from the outside, thus avoiding the problem of frosting in the heat exchanger.
The automotive industry has begun to shift to new energy electric vehicles due to the challenges of oil and environmental resources. However, heating in low-temperature environments results in poor endurance of electric vehicles in winter, which has always hindered the development of electric vehicles. The main reason is that the commonly used heating method, Positive Temperature Coefficient Heating (PTC), has a lower efficiency, while traditional heat pumps lack heat absorption and heat exchanger frosting, resulting in heat pumps not being able to be used well in low-temperature environments. This article proposes a transcritical CO2 self-enhanced enthalpy heat pump system suitable for low-temperature environmental applications to address this issue. The system can be divided into self-enhanced enthalpy heat pump (SEHP) mode, self-enhanced enthalpy waste heat hybrid heat pump (SEWHHP) mode, and waste heat recovery heat pump (WHHP) mode based on no waste heat, low waste heat to high waste heat. A mathematical analysis was conducted on the operating characteristics of CO2 in three modes using the AMESIM platform. The heat from the condenser in the system can be distributed by the cooling water circuit to the cabin and evaporator for heat absorption, and the waste heat from the battery motor can also be well utilized by the evaporator. This article focuses on analyzing the efficiency of three modes under different ambient temperatures, target temperatures, and residual heat. When the ambient temperature is −20℃, the mode switches from the SEHP to the SEWHHP mode and then to the WHHP mode, with the efficiency increasing from 0.89 to 1.31 and 1.77, respectively. COP increases with increasing ambient temperature in all modes, with the WHHP mode increasing from 1.46 to 1.67 when the ambient temperature increases from 50 °C to 20 °C. The efficiency is much greater than that of PTC, and this study provides new ideas for the thermal management of electric vehicles in low-temperature environments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The aspect ratio of manifold microchannel was optimized at fixed mass flux.•The manifold microchannel has the optimal aspect ratio at fixed mass flux.•The optimal aspect ratio was analyzed at ...various working fluids and solid materials.•A criterion formula of optimal aspect ratio was given.
In order to provide a basis for the design and optimization of the aspect ratio of the manifold microchannel, the effect of channel aspect ratio on the heat transfer performance of manifold microchannel heat sinks at fixed mass flow rate is numerically investigated in this present work. The results show that the heat transfer performance of the manifold microchannel heat sink reaches the peaks at one specific channel aspect ratio, which is defined as optimal aspect ratio αo. It is found that the optimal aspect ratio is different at various working fluids and solid materials. The optimal aspect rises with the increase of thermal conductivity of the channel wall and dynamic viscosity of working fluid, or the decrease of specific heat capacity of working fluid and thermal conductivity of working fluid. Further numerical simulations are conducted to quantitatively analyze the influence of each thermophysical parameter on αo. Finally, a criterion formula is given to describe the optimal aspect ratio as a function of Re number, Pr number, and the ratio of thermal conductivity between working fluid and channel wall.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•A model for the anisotropic thermal conductivity of CPCMs under compression.•Compression changes the directional distribution of thermal conductive particles.•The radial thermal conductivity ...increases with the increase of compression.•The axial thermal conductivity decreases with the increase of compression.•Compression has a little effect on axial heat transfer coefficient of composites.
Anisotropic composite phase change materials (CPCMs), as thermally conductive materials, are typically compressed to reduce contact thermal resistance in thermal control systems. However, the effect of compression on the anisotropic heat transfer properties of CPCMs is rarely studied. Therefore, a model for determining the anisotropic thermal conductivity of thermally conductive CPCMs under compression is proposed. The anisotropic thermal conductivity of CPCMs under different compression ratios is calculated by combining the directional distribution of thermal conductive particles and the anisotropic third-order T-matrix approximation (ATA) equation. Under different compression ratios, the model's calculated values for CPCMs are in good agreement with the experimental values. According to the model analysis, the radial thermal conductivity of the composite increases and the axial thermal conductivity decreases as the compression ratio increases from 0 to 50 %. Additionally, as the mass fraction of thermally conductive particles increases from 5 % to 30 %, both the radial and axial thermal conductivities of the CPCM increase. When the mass fraction of thermally conductive particles is low, the axial heat transfer coefficient of thermally conductive CPCMs increases as the compression ratio increases. When the particle mass fraction is higher than 20 % and lower than 30 %, the effect of the compression ratio on the axial heat transfer coefficient is very small.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•An embedded manifold interlaced pin-fin microchannel heat sink was designed and fabricated.•Under different working conditions, the flow and heat transfer characteristics of staggered pin-fin ...microchannels and rectangular microchannels are compared.•The staggered pin-fin microchannel provides stronger turbulence effect and better heat transfer performance.•The bubble accumulation at the end of the pin fin affects the convective heat transfer and pump power.
High-heat-flux thermal management of high-power chips in electronic devices is becoming more and more critical. Manifold microchannel heat sink is one of the effective choices. In order to improve the heat dissipation capacity of manifold microchannel heat sink, a silicon-based thermal test chip including staggered pin-fin microchannels is designed and fabricated. Staggered pin-fin microchannels and rectangular microchannels with the same area size are embedded on the opposite side of the silicon-based thermal test chip for liquid-cooled heat dissipation. The experiments are conducted using HFE7100 as the working fluid. The heat transfer performance and the flow characteristic of two distinct microchannel structures are evaluated. It is found that the differences in temperature, convection heat transfer coefficient and pressure drop caused by different microchannel structures are greatly related to the mass flow rate. The heat transfer performance of the staggered pin-fin microchannel heat sink is better than that of the rectangular microchannel heat sink at a larger mass flow rate, but it also brings greater flow pressure drop. The bubble effect caused by the transition from single-phase to two-phase flow in microchannels makes the monotonicity of the convection heat transfer coefficient decrease significantly and the monotonicity of the pressure drop increase significantly. Compared to rectangular microchannel heat sinks, staggered pin-fin microchannel heat sinks reach the two-phase flow at higher heat flux. At the mass flow rate of 5 mL/s and the heat flux of 700 W/cm2, the staggered pin-fin microchannel heat sink can reduce the chip surface temperature by 8 K compared with the chip surface temperature of the rectangular microchannel heat sink.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Abstract
The two-phase spray cooling system has the characteristics of high cooling medium utilization, strong cooling capacity, and less space, which can meet the increasing heat dissipation demand ...of electronic components. This paper established a two-phase spray heat transfer system with electron-fluorinated liquid perfluoro-4-methyl-2-pentene (FCM-47) as the experimental liquid. The influence of spray distance, nozzle flow characteristic coefficient, and atomization pressure difference on the spray characteristic and the heat dissipation performance of spray cooling in the boiling zone were studied. The experiment shows that the critical state will occur in advance when the spray distance is reduced to a certain height (3 mm). An optimal spray distance of 8 mm corresponds to the lowest surface temperature and maximum CHF (critical heat flux). Increasing the nozzle flow characteristic coefficient and spray pressure difference can improve the heat exchange by increasing flow. However, increasing the nozzle flow characteristic coefficient can increase CHF but has little effect on the critical temperature, and increasing the spray pressure difference can increase both the critical temperature and CHF. This is related to their effect on atomization characteristics.
