查看更多>>摘要：Heat recovery technologies are used to reduce the energy use and the operating costs for ventilation systems in buildings. Run-around heat recovery systems for ventilation are commonly used in buildings when cross-contamination between the air streams is not acceptable, in buildings with complex ducting and in retrofit projects with space limitations. The design and operation of run-around systems are rather complex, especially in ventilation systems with variable air flow rates since the coupling liquid flow rate must be adjusted with respect to the air flow rate.
查看更多>>摘要：Loop Heat Pipe (LHP) is a passive phase-change heat transfer device. Among other factors, its performance largely depends on the heat leak from the evaporator to the compensation chamber (CC). For copper wick LHP, a high heat leak creates a problem in successful LHP start-up and can adversely affect its operation. The present study focuses on reducing the heat leak through manipulation of the copper wick properties. The effect of fluid charging, overall pressure drop, and wick oxidation on thermal performance is investigated. A cylindrical copperacetone LHP is tested in favorable orientation for two cases with (1) oxygen-free, or pure copper wick and (2) oxidized copper wick in its evaporator. For Case #1, LHP is filled with acetone at different Charging Ratios (CR) of 50%, 60%, and 70%. For Case #2, the charging ratio is kept constant at 50%. For the pure copper wick, a charging ratio of 50% provides the best thermal performance, and it decreases with an increase in charging ratios. At CR = 50%, LHP can transfer a heat load of 90 W (hEvp = -900 W/m2K) and 180 W (hEvp = -2500 W/ m2K) at the evaporator temperature below 100 degrees C for the pure copper and oxidized wick, respectively. The significant improvement in LHP thermal performance of the latter is attributed to a decrease in heat leak because of the low thermal conductivity of the oxidized porous wick. The heat leak for pure and oxidized wick is found -18% and -7% of input heat loads, respectively. This can be an effective methodology to pave the way for the usage of commercial copper-based LHPs for the thermal management of terrestrial devices.
查看更多>>摘要：This study investigates cold thermal energy storage (CTES) using a helical coil heat exchanger modified with bubble injection. One of the effective methods for increasing the heat transfer rate in heat exchangers is using bubble injection. A helical coil heat exchanger is immersed inside a cylindrical water storage tank, where the helical coil is the evaporator of a vapor compression refrigeration cycle (VCRC) and provides the designed cooling. Experimental studies were carried out to examine the impact of bubble injection on Nusselt number, the temperature differences in the storage tank, exergy degradation in the evaporator, and cycle coefficient of performance (COP). The bubbles were injected from the bottom of the storage tank in four different geometries at airflow rates ranging from 3 to 11 L/min. The experimental results of this study revealed that bubble injection could significantly increase the COP and heat transfer rate from the storage tank, as well as the exergy destruction and Nusselt number (Nu). This increase was highly dependent on the geometry and flow rate of the bubble injection. The results also indicated that the bubble injection has an optimal flow rate value, which was 9 L/min in this study. More specifically, the COP of the refrigeration cycle and the Nu number increased by 124% and 452%, respectively, compared to the non-bubble injection mode. Finally, for calculating the outside Nusselt number of the helical coil, an empirical correlation as a function of bubble flow rate and bubble injection angle was proposed.
查看更多>>摘要：Microstructured surfaces with spatial arrangements of hydrophilic and hydrophobic regions have been widely studied in boiling heat transfer enhancement. In this study, a conceptual design of microstructured surfaces with time-varying wettability is proposed to regulate bubble dynamics and thus enhance the heat transfer performance in the nucleate boiling regime. The effectiveness of the design is examined numerically for single-bubble heat transfer on a micropillar-arrayed silicon surface using a comprehensive three-dimensional model. The intrinsic contact angle is initially set as 48 degrees, it is subsequently reduced to 20 degrees in the bubble growth (0.152 and 0.4 ms) or departure stage (0.60 and 0.80 ms). The simulations show that two mechanisms contribute to heat transfer enhancement. First, the time-varying wettability decreases the adhesion force on the bubble exerted by the surface, thereby increasing the bubble departure frequency. Second, it also increases the local curvatures of the bubble, leading to the formation of thin liquid films between the bubble and micropillar walls. The liquid films suppress dry spots and hence enhance the heat removal of the bubble from the surface. The simulations also show that the reduction in the intrinsic contact angle enhances the evaporation on the liquid-vapor interface but deteriorates the evaporation in the microlayer. Because the former dominates the heat transfer process of the bubble on the micropillar-arrayed surface, the time-varying wettability increases the total evaporation rate. In the meantime, the time-varying wettability reduces the time proportion of the departure stage, which shortens the weakened heat transfer period in a bubble life-cycle and thus enhances the average heat transfer rate. As compared the case with constant wettability, the average heat transfer rate for the cases with time-varying wettability increases by 38.5%, 23.1%, 19.2%, and 15.4%, respectively, when the intrinsic contact angle is reduced at 0.152, 0.40, 0.60, and 0.80 ms, indicating that earlier wettability modulation gives rise to more significant heat transfer enhancement. The optimal time for wettability modulation is exactly the same as the time at which the total evaporation rate begins to decline.
查看更多>>摘要：For the aim of increasing the heat transfer enhancement, a hybrid method in which active and passive heat recovery techniques have been used together. The usage of nanofluid, MHD and dimpled fins tube have not been utilized together so far. Regarding this issue, this study is the first numerical study to determine effect of usage of three effects together comprehensively. In this study, thermo-hydraulic performance of Fe3O4/H2O nanofluid (ferro-nanofluid) flow inside dimpled tube under magnetic field effect has been examined numerically. The main purpose of the study is to obtain numerical data for turbulent flow in the spherical dimpled tubes providing some aid to design a highly efficient thermal energy storage devices. Dimple geometry with nondimensional pitch ratio (P/d = 3.75, 7.50 and 11.25), Hartmann number (Ha = 75, 150, 225) and nanoparticle volume fraction (phi = 0.5, 1.0 and 2.5 vol%) are the parameters investigated in this study. The numerical analyses have been carried out Reynolds number ranging from 10,000 to 50,000 at a constant heat flux at 20 kW/m2. The simulations have been built up by Realizable k-epsilon turbulence model and single-phase approach. Also, "MagnetoHydroDynamic" (MHD) module has also been activated for defining magnetic field effect. The results showed that Nusselt number increases with increasing Reynolds number and decreasing pitch ratio. The dimple geometry type of P/d = 7.50 has been determined as the most efficient dimple geometry type. In the case of highest magnetic field intensity, the highest Nusselt number increment (72.48%) has been obtained for phi = 2.5 vol% compared to the base fluid of distilled water using as the working fluid for smooth tube. The highest PEC value was also obtained as 1.126 for the case of P/d = 7.5, phi = 2.5 vol% and Ha = 75. In addition, the effect of magnetic field intensity on velocity and temperature distributions has been presented with contour graphs.
查看更多>>摘要：In this study, a new heat dissipation device with a composite heat sink of a loop heat pipe (LHP) system was evaluated for efficient cooling of electronic components. To improve the heat dissipation ability of the high heat flux chip and its local hot spot area, LHP, which had a gas-liquid phase change as the main mechanism of heat dissipation, was selected to remove heat. The experimental results showed that the average temperature performance was improved after the graphene copper foil heat sink was fitted, and the maximum temperature under the heat loads of 80 W, 90 W, 100 W, and 120 W was reduced by 3.8 degrees C, 5.7 degrees C, 6.5 degrees C, and 7.7 degrees C, respectively, compared with without a heat sink. The hot spot temperature increased linearly with an increase in the inlet water temperature. Increasing the ambient temperature within a certain range had little effect on the heat-dissipation effect of the hot spots. To interpret the experimental results, the hot spot heat transfer performance of the evaporation section of the LHP with a composite heat sink was set up by the equivalent thermal resistance model. The CFD simulated results were in good agreement with the experimental values. This indicated that the heat sink significantly reduced the hot spot temperature and improved the cooling uniformity largely. When the transverse thermal conductivity of the heat sink is sufficiently high, the increase in longitudinal thermal conductivity is more effective in reducing the hot spot temperature than the transverse thermal conductivity. Moreover, it has been suggested that the hot spots and heating power play a major role in the actual electronic structure and heat dissipation design. In summary, the study proposes a valuable method for improving the uniform cooling of chips, which was conducive to the heat management of the system.
Chang, MengzhaoYu, Young SooPark, SungwookPark, Suhan...
12页
查看更多>>摘要：In the gasoline direct injection (GDI) engine, spray atomization contributes significantly to the formation of a uniform mixture and the effective combustion of the spray. In this study, a phase Doppler particle analyzer (PDPA) was used to measure the spray atomization characteristics of two five-hole GDI injectors with different hole arrangements under subcooled and mild flash-boiling conditions (superheated degree = 20 degrees C). Based on the spray patterns of the two injectors on a plane 30 mm below the injector tip, the two injectors were called triangle-patterned and pentagon-patterned injectors. A total of 100 measuring points were selected on a plane 30 mm below the injector tip, and the spray droplet diameter and velocity at these points were measured to understand the overall atomization characteristics of the spray in space. The experimental results indicated that as the spray transitioned from subcooled to flash boiling conditions, a polarization phenomenon occurred in which the velocity in the center of the plume increased and the velocity in the peripheral region of the plume decreased. However, the spray droplet diameter significantly decreased under flash-boiling conditions. The average Sauter mean diameter (SMD) of the two injectors on a plane 30 mm below the injector tip decreased by approximately 4 mu m, and the uniformity of the spatial distribution of the SMD also increased. Compared with the triangle patterned injector (injector A), the average SMD of the pentagon-patterned injector (injector B) was slightly smaller; this was attributed to the stronger air entrainment inside the spray and the enhanced air flow caused by the larger plume spacing of the pentagon-patterned injector.
查看更多>>摘要：Jet impingement are widely used in the piccolo hot air anti-icing chambers of aircraft. The high pressure at the nozzle and low pressure in the chamber lead to a relatively large nozzle pressure ratio, triggering an unexpanded jet state. The unexpanded jet impingement significantly differs from the subsonic jet impingement in flow and heat transfer. Therefore, this study conducts numerical investigations on the flow and heat transfer characteristics of unexpanded jet impingement. In addition, the effects of nozzle pressure ratio (NPR) and dimensionless impinging distance (L/d) are considered. According to the numerical results, the impinging air cannot be fully expanded in the free jet zone, and will continue to expand after entering the wall jet zone, triggering an abrupt drop in the hot air temperature, under a small dimensionless impinging distance (L/d = 2.5 and L/d = 5.0). In this state, the hot air will not heat the surface effectively, as the surface will heat the air reversely. Furthermore, it is also determined that the ambient temperature should be included as an influencing factor in the analysis of jet impinging heat transfer. For high-speed jet impingement, especially unexpanded jet state, the ambient air will influence the total temperature of jet boundary due to the large difference of static temperature between free jet zone and chamber environment.
查看更多>>摘要：To improve the temperature uniformity and cooling performance of the battery module, a hybrid battery thermal management system (BTMS) with liquid cooling and phase change materials (PCM) containing different expanded graphite contents is proposed. To adjust the heat transfer efficiency of composite phase change material (CPCM) along the direction of liquid flow, the segments of CPCM matrix contain different expanded graphite (EG) contents. Subsequently, the effects of the layout of CPCM matrix, length distribution of each segment, the structure parameters and cooling strategy on cooling performance of the battery module are investigated using computational fluid dynamics (CFD) model at 4C discharge rate and ambient temperature of 308.15 K. The results demonstrate that the maximum temperature (T-max) and temperature difference (Delta T) of BTMS adopting segmented layout III are significantly reduced by 1.3 K and 1.4 K respectively compared with layout I. Additionally, the layout III exhibits optimal cooling performance when length of each segment is 110 mm, 120 mm and 120 mm, respectively. Moreover, the T-max and Delta T decrease with increase of cell-to-cell spacing (L) and diameter of liquid channel (d), and Delta T is only 2.2 K under the condition of L(24)d(5) (L = 24 mm, d = 5 mm). During the C-1-charging and C-4-discharging cycles, the Tmax of BTMS with normal strategy remains at 319.5 K and Delta T lower than 3 K; furthermore the strategy of delayed liquid cooling can significantly reduce power consumption by 33.3 % without sacrificing the cooling performance.
查看更多>>摘要：In the past, fin-and tube heat exchanger (FTHE) tube pattern ratios have been largely based on ad-hoc design principles. Here, we investigate the optimal tube arrangements for a FTHE with plain fins in marine environments represented by two different air types; one for unfiltered air with high condensation rate and one for clean dry filtered air conditions. The thermal-hydraulic efficiency of the FTHE design is measured by comparing a modified ratio of Colburn j-factor and Fanning friction factor. The regression model generated from the CFD data is then used to identify the maximum efficiency for two design specific fin pitches separately. We identified two optimal tube patterns: one for a large fin pitch for unfiltered air, and another for a small fin pitch for filtered air. Manufacturing restrictions were found to significantly limit the maximum achievable efficiency of a tube pattern. By neglecting the related manufacturing restrictions, 4% higher efficiency for a fin pitch of 1.5 mm and 23% higher efficiency for a fin pitch of 3.5 mm is achieved. Without any application specific limitations or manufacturing restrictions the fin pitch 1.5 mm can have a 36% increased efficiency than fin pitch 3.5 mm. These novel results show that development in manufacturing have potential for significant improvements in thermal-hydraulic efficiency.
NSTL
Elsevier