International Journal of Heat and Mass Transfer最新文献

In this study, the effects of forced convection heat transfer on the surfaces of pin plate heat exchangers with different geometries modified to the heat exchanger form were experimentally investigated. A total of 8 pin fin plate heat exchangers, four of which are staggered and the other four are in-line, were used in the experiments. The heat transfer surface areas of the pin plate heat exchangers are produced equally. The only difference is their geometry, which is produced in four different shapes: triangle, circle, ellipse and square. The experiments were carried out at air velocities of 1 to 6 m/s with 1 m/s increments and at 10 W and 50 W input powers with 10 W increments. The staggered configuration of the pin fin plates demonstrated high performance in heat transfer. The study found that heat exchangers with cornerless pin plates have lower heat transfer compared to those with cornered pin plates. The pressure drops increased as the velocity of the coolant air increased, and the highest-pressure drop was observed in staggered triangular and elliptical pin plate heat exchangers. This research is considered innovative as it involves an experimental comparison between staggered and in-line pin plate heat exchangers in four different geometries, all with equal surface area.

The thermal characteristics in a double-layered 3D-CLPHP are investigated by visualization experiments. The results are compared with those of a single-layered CLPHP under the vertical and horizontal orientations and a filling ratio (FR) of 35%, 50%, or 65%. The tube's inner diameter (ID) is 6 mm, slightly over the limiting value for water. The orientation is found highly determinative to the flow pattern of liquid slug trains in each tube layer. The flow behavior appears similar for the two layers under the vertical orientation but apparently different under the horizontal orientation. When horizontally placed, the liquid slug trains tend to drain down to the lower layer, thereby not only triggered but sustained continuous pulsation motion. Intense cross-layered flow motion, attributed to the interaction between the downward gravity and upward gaseous expansion or buoyancy effect, is recorded during the operation. Sufficient working fluid distribution inside each layer for the overall FR of 65% is recommended. Instead, the single-layered CLPHP fails to maintain a stable oscillation motion without the assistance of gravity. The double-layered CLPHP outperformed the single-layered CLPHP by 12.8$-$15.1% in thermal resistance even under the vertical orientation.

Considering the safety and effectiveness of lithium-ion batteries for new-energy vehicles under extreme working conditions, a topology optimization design method based on a bionic leaf-vein structure is proposed in this paper. Taking the liquid cooling plate for a lithium-ion battery as the research object, heat dissipation channels with a bionic leaf-vein structure were designed. The number, angle, width, and height of initial cold plate (ICP) were analyzed through orthogonal experiments. The optimized cooling plate (OCP) with a bionic leaf-vein structure was obtained by solving with the non-dominated sorting genetic algorithm-II (NSGA-II). Then, the two-dimensional structure of the OCP was used as the initial solution, and topology optimization was performed with an initially uniformly distributed density field. Maximum heat transfer and minimum dissipative work were used as the multi-objective functions to obtain the bionic topological cooling plate (BTCP) and the topological cooling plate (TCP). Finally, the performance of the BTCP and TCP were compared with that of the OCP. The results showed that the OCP has better heat dissipation compared to the ICP, with the maximum temperature (T_max) reduced by 1.06 °C and maintained around 33 °C. Additionally, the pressure drop (ΔP) is reduced by 40.03%, and the standard temperature difference (T_σ) is reduced by 8.98%. The T_max of the BTCP was reduced by 0.71 °C compared to that of the OCP. Furthermore, the ΔP and T_σ are reduced by 71.25% and 40.79%, respectively. Compared with the TCP, the thermal homogeneity of the BTCP increases by 29% even though the ΔP increases by 2.87 Pa. Analysis of the comprehensive indexes shows that the performance of the TCP and BTCP improves by 80% and 96%, respectively, on the basis of that of the OCP. Moreover, the BTCP features a better channel structure, which ensures thermal homogeneity and saves computation time of the model.

Thermophoresis describes mass transport in a non-isothermal temperature field and thus provides a fundamental understanding of the behavior of colloidal particles. Various methods have been proposed for measuring the Soret coefficient, a representative value of thermophoresis. In particular, microscopic channels are an emerging method as they shorten the equilibrium time and allow direct observation of the particles. However, little emphasis has been placed on the simultaneous consideration of fluid dynamics, heat transfer, and mass transfer characteristics within the microfluidic channel, despite the simultaneous presence of natural convection and thermodiffusion phenomena. In this study, we present a novel approach to address this gap by introducing a figure of merit, which incorporates essential parameters to accurately characterize a specific cell configuration. This figure of merit allows for the identification of a reliable measurement range in a microfluidic channel with a temperature gradient, while accounting for fluid dynamics, heat transfer, and mass transfer characteristics. The proposed approach is validated through rigorous simulations and experiments, enabling an evaluation of the impact of figure of merit-derived parameters on the measurement channel. The findings from our study demonstrate that the figure of merit serves as a representative measure for stable thermophoretic measurements in a microfluidic channel. Moreover, we propose a threshold value that signifies the transition from a diffusion-dominant to a convection-dominant field.

The paper presents an experimental investigation of the thermo-hydraulic performance of a prototype single-plate minichannel heat exchanger. The symmetric heat exchanger plate made of aluminum alloy consists of square cross section channels with a hydraulic diameter of 1000 µm and a length of 100 mm and rectangular inlet and outlet plenums. The heat exchanger operates in a counterflow configuration and the working fluids are hot and cold deionized water, respectively. The flow in minichannels is considered laminar due to its maximum Reynolds number limited to approximate value of 2200. Experimental measurements were carried out for various combinations of hot and cold side Reynolds numbers for the purpose of seeking optimal thermo-hydraulic performance. The minichannel geometry and operating conditions were chosen to operate in the laminar thermal entry length in order to achieve higher average Nusselt number along the minichannel length compared to the Nusselt number in the fully developed laminar flow. The general analytical model for prediction of the heat transfer performance of these types of heat exchangers is presented and includes the heat transfer process in the inlet plenum, the parallel configuration of the minichannels, and the outlet plenum. A comparison of the overall heat transfer coefficient resulting from the experiment and the analytical model is presented. The thermo-hydraulic performance of the investigated minichannel heat exchanger and exemplary microchannel heat exchangers is also presented. The main advantage of the presented minichannel heat exchanger is a very high overall heat transfer coefficient up to 3000 W/(m²K) and relatively low pressure drop on the one side of the heat exchanger up to 6.5 kPa, while still maintaining the high compactness and ease of manufacturing compared to microchannel heat exchangers. The comparison of minichannel heat exchanger considered in this paper and exemplary microchannel heat exchangers reflected slight decrease of the overall heat transfer coefficient, while still being approximately of the same order, together with incomparably lower pressure drop, on average of two orders of magnitude.

A flat-type 10 kW-class loop heat pipe (LHP) with a box-type wick was designed and developed to handle the higher heat loads in waste heat recovery applications, such as industrial processing and internal combustion engines. Additionally, a numerical model was developed to predict the overall thermal performance of the LHP. The LHP used a stainless steel wick with a pore diameter of 2.0 µm and pure water as the working fluid. The LHP was tested using two types of cooling for the condenser (forced and natural convection), two types of evaporator orientations (horizontal and vertical), and with and without gravity assist (0.3 m and 0 m). For all the tests, the maximum heat load ranged from 5 kW to 10 kW, with the test with a gravity assist of 0.3 m, vertical evaporator orientation, and natural convection performing the best. This test sustained a 10 kW heat load at a steady-state temperature of 182 °C for the evaporator. During the same test, a maximum evaporative heat transfer coefficient of 92,000 W/m²/K at 4.5 kW and a thermal resistance between the evaporator and condenser value of less than 0.007 K/W was achieved. A numerical model was developed to compare the experimental results with the numerical temperature results for the heater block, evaporator, vapor line, condenser, liquid line, and compensation chamber. Overall, the average temperature difference for all components ranged from 7.1 °C to 10.6 °C, with the horizontal orientation without gravity assist and natural convection test predicting the best. The findings demonstrate that LHPs can handle the higher heat loads that are found in waste heat recovery applications for industrial processing and internal combustion engines.

The study deals with understanding the flow and heat transfer aspects of a multi-orifice synthetic jet (SJ), and the results are compared with a single orifice SJ. The effect of actuation frequencies and amplitude is studied for a single orifice SJ, to understand the formation and the development using smoke-wire visualization. The generation of vortex structure is visualized using the smoke to understand the difference between the formation of single and multi-orifice SJ. Further, a particle image velocimetry (PIV) technique is applied to study the evolution of the free SJ. The mixing and spreading characteristics of SJ are investigated using vorticity and mean velocity profiles in the flow field for single and multi-orifice cases. Heat transfer experiments on a flat surface are carried out for both the orifice types. The results of the study show that large-sized vortices are produced with a decrease in the frequency and an increase in the amplitude of the SJ actuator. While increasing the frequency, the vortices are more closely spaced in the flow field, and with an increment in amplitude, vortices are placed at larger distances. The PIV results indicate that the multi-orifice produces strong mixing and vorticity in the near-field region, while it loses strength quickly after a certain distance downstream. For the single-orifice case, the development and mixing of the jet are fairly gradual compared to the multi-orifice case. All these flow characteristics point towards higher heat transfer rates at lower surface spacings, which is reinforced by the heat transfer results. The multi-orifice configuration provided up to 30 % improvement in the average heat transfer from the surface compared to the single-orifice case of equivalent diameter. This work adds to our understanding of multiple orifice and impinging jets and provides a link between heat transfer and fluid flow, leading the way for thermal management systems in confined spaces.

CO₂/propane mixtures benefit the safe operation of the supercritical Brayton cycle while improving economic efficiency and mitigating environmental impact. Due to the intermittency of energy resources, understanding the dynamic characteristics of the supercritical mixture Brayton cycle is essential. The transition time of the cycle depends on the total time of the unsteady-state heat transfer. As a typical heat exchanger, the transient analysis of the printed circuit heat exchanger (PCHE) is necessary and significant. In this paper, the dynamic response of the straight PCHE in the supercritical CO₂/propane mixture Brayton cycle is thoroughly studied. First, the dynamic behavior of the mixture-mixture PCHE is analyzed when the inlet temperature or mass flow rate abruptly changes. Furthermore, the equilibrium times of the mixture-mixture PCHE under different disturbances are compared. CO₂/propane mixtures are more favorable for the stability of the parameters (outlet temperature and pressure drop). The mixture-mixture PCHE exhibits better flow and heat transfer properties than the CO₂CO₂ PCHE. Compared to CO₂, CO₂/propane mixtures could reduce the equilibrium time by more than 32 % when the molar fraction of propane is equal to 0.5. However, the outlet temperature of the mixtures in the hot channel is higher than that of CO₂, because the energy is not fully utilized.

Serving as the regulatable external force, the magnetic nanoparticles induced force inside the magnetic field can be an efficient way to handle the heat storage of phase change materials. The deep understanding on the relation between the thermomagnetic convection and externally applied gradient magnetic field is no doubt essential. In this paper, the impacts of gradient magnetic field on the melting process of Fe₃O₄-paraffin wax composite phase change materials are identified through a numerical simulation. Particularly, the effects of some key parameters of nanoparticle concentration, heating surface temperature, and gradient magnetic field strength on the heat storage performance of phase change materials are discussed in depth. It is found that the angle (θ) between the volumetric force and the normal direction of the heating surface dominates the phase change heat storage effect in the cavity. Compared to the case with only gravity-force (θ = 90°), the cases with θ larger than 90° can enhance the phase change heat storage process, while those with θ less than 90° weakens it. Note that when the magnetic field gradient is in the same direction as the heat transfer direction, the heat storage performance of the phase change material decreases and vice versa increases. Moreover, the increase of heating surface temperature, nanoparticle concentration, and magnetic field strength can positively accelerate the heat storage process. The gradient magnetic field of 0.8 and -0.8 T·m⁻¹ would decrease and increase the specific volume heat storage power by 26.49 % and 29.04 % in sequence, compared to the no magnetic field case.

We experimentally investigate the near-wall heat transfer at single bubble growth in nucleate saturated pool boiling of water at atmospheric pressure. Our focus is on the evaporation of the micro-metric thin film of liquid (microlayer) that is formed between the heating wall and the bubble. High speed and high resolution optical techniques are employed. Synchronous and simultaneous measurements of the microlayer thickness, wall temperature and bubble macroscopic shape are performed by white light interferometry, infrared thermography and side-wise shadowgraphy, respectively. We measure the wall temperature of an ITO heating film through a transparent to the infrared waves porthole. The heating is provided by an infrared laser. The wall heat flux is numerically reconstructed by using the experimental wall temperature data. We reveal a temporal rise of the thermal resistance of the liquid–vapor interface during the microlayer evaporation, which corresponds to a decrease of the accommodation coefficient. We attribute it to the progressive accumulation of impurities at the interface during evaporation. The contribution of microlayer evaporation to the overall bubble growth is about 18%.