Fully developed thermal behaviors for parallel flow microchannel heat exchanger

The hydrodynamics and thermal behaviors of the flow in micro-heat exchanger parallel-plate are investigated numerically. The model that combines both the continuum approach and the possibility of slip at the boundary is adopted in the study. Effects of different parameters, such as, Knudsen number (Kn), heat capacity ratio (Cr), the effectiveness (e) and number of transfer units (NTU) are investigated. It is found that both the velocity slip and the temperature jump at the walls increase with increasing Kn. On the other hand, at low values of Kn, the NTU increases with increasing Cr, but at high values of Kn the NTU decreases with increasing Cr. With regard to the effect of Cr on e, it is found that increasing Cr leads to a reduction in e and for all Kn.     

Cooling performance of a microchannel heat sink with nanofluids

In this paper, the cooling performance of a microchannel heat sink with nanoparticle–fluid suspensions (“nanofluids”) is numerically investigated. By using a theoretical model of thermal conductivity of nanofluids that accounts for the fundamental role of Brownian motion, we investigate the temperature contours and thermal resistance of a microchannel heat sink with nanofluids such as 6 nm copper-in-water and 2 nm diamond-in-water. The results show that the cooling performance of a microchannel heat sink with water-based nanofluids containing diamond (1 vol.%, 2 nm) at the fixed pumping power of 2.25 W is enhanced by about 10% compared with that of a microchannel heat sink with water. Nanofluids reduce both the thermal resistance and the temperature difference between the heated microchannel wall and the coolant. Finally, the potential of deploying a combined microchannel heat sink with nanofluids as the next generation cooling devices for removing ultra-high heat flux is shown.     

Thermal and hydraulic characteristics of nanofluid flow in a helically coiled tube heat exchanger

The effects of using different geometrical parameters with the combination of nanofluid on heat transfer and fluid flow characteristics in a helically coiled tube heat exchanger (HCTHE) are numerically investigated. A CuO nanoparticle with a diameter of 25 nm dispersed in water with a particle concentration of 4% was used as the working fluid. The three dimensional governing equations (continuity, momentum and energy) along with the boundary conditions are solved using the finite volume method (FVM). The mass flow rate of water in the annulus was kept constant and the nanofluid flow rate in the inner tube was varied. The effect of flow configuration (parallel and counter) was also examined in this study. The performance of the HCTHE was evaluated in terms of Nusselt number, heat transfer rate, pressure drop, effectiveness and performance index. The results reveal that certain geometrical parameters such as the helix radius and inner tube diameter do affect the performance of the HCTHE under laminar flow conditions. It is also found that counter-flow configuration produced better results as compared to parallel-flow configuration.     

Numerical study of flow and heat transfer characteristics of alumina-water nanofluids in a microchannel using the lattice Boltzmann method

In the present study, mathematical modeling is performed to simulate force d convection flow of Al₂O₃/water nanofluids in a microchannel using the lattice Boltzmann method (LBM). Simulations are conducted at low Reynolds numbers (Re ≦ 16). Results indicate that the average Nusselt number increases with the increase of Reynolds number and particle volume concentration. The fluid temperature distribution is more uniform with the use of nanofluid than that of pure water. Furthermore, great deviations of computed Nusselt numbers using different models associated with the physical properties of a nanofluid are revealed. The results of LBM agree well with the classical CFD method for predictions of flow and heat transfer in a single channel and a microchannel heat sink concerning the conjugate heat transfer problem, and consequently LBM is robust and promising for practical applications.     

Influence of various base nanofluids and substrate materials on heat transfer in trapezoidal microchannel heat sinks

Numerical investigations are performed to investigate the laminar flow and heat transfer characteristics of trapezoidal MCHS using various types of base nanofluids and various MCHS substrate materials on MCHS performance. This study considered four types of base fluids including water, ethylene glycol (EG), oil, and glycerin with 2% volume fraction of diamond nanoparticle, and four types of MCHS substrate materials including copper, aluminium, steel, and titanium. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite volume method. It is found that the best uniformities in heat transfer coefficient and temperature among the four mixture flows can be obtained using glycerin-base nanofluid followed by oil-base nanofluid, EG-base nanofluid, and water-base nanofluid heat sinks. However, the heat transfer performance of water-base nanofluid can be greatly enhanced in steel made substrate heat sink.     

Influence of transverse vibrations on convective heat transfer in parallel flow tube-in-tube heat exchanger

Tube-in-tube heat exchangers are widely used in food processing industries and wastewater treatment for both heating and cooling. Enhancement techniques namely active, passive, and compound are developed to reduce the thermal resistance in heat exchangers by improving convective heat transfer with or without increase in surface area. The present experimental study is aimed at analyzing the influence of vibrations on the convective heat transfer of a parallel flow tube-in-tube heat exchanger. The heat exchanger is placed in horizontal position and is subjected to transverse vibrations under turbulent fluid flow conditions. Experiments were performed at four frequencies (20, 40, 60, and 100 Hz), three amplitudes (1, 2, and 3 m/s²), and three vibration generator positions along its length, in the Reynolds number range of 10 710 to 21 420. An enhancement in Nusselt number is found with vibration than without vibration throughout the entire range of Reynolds numbers. A maximum enhancement of 33% at 40 Hz frequency, 3 m/s² amplitude, and vibration generator position at three-fourth of the tube length was observed. Empirical correlations are developed for Nusselt number to determine the heat transfer coefficient with vibration with an error of $\pm$10%.     

Heat transfer enhancement of nanofluids flow in microtube with constant heat flux

In this paper, laminar convective heat transfer in a two-dimensional microtube (MT) with 50 μm diameter and 250 μm length with constant heat flux is numerically investigated. The governing (continuity, momentum and energy) equations were solved using the finite volume method (FVM) with the aid of SIMPLE algorithm. Different types of nanofluids Al₂O₃, CuO, SiO₂ and ZnO, with different nanoparticle size 25, 45, 65 and 80 nm, and different volume fractions ranged from 1% to 4% using ethylene glycol as a base fluid were used. This investigation covers Reynolds number in the range of 10 to 1500. The results have shown that SiO₂–EG nanofluid has the highest Nusselt number, followed by ZnO–EG, CuO–EG, Al₂O₃–EG, and lastly pure EG. The Nusselt number for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nusselt number increases with Reynolds number.     

Thermal performance of a single-row fin-and-tube heat exchanger

Experiments were carried out to study the heat transfer characteristics of a single-row aluminum fin-and-tube crossflow heat exchanger with an emphasis in the regime of low flow rate of the in-tube fluid. The Chilton-Colburn analogy, in conjunction with the least-squares power-law technique, was used to correlate experimental data. Both air- and water-side heat transfer correlations were developed in the form of the Nusselt numbers as a function of Reynolds and Prandtl numbers. The experimental observations are quantitatively compared to the predictions of correlations available in the published literature. Different transfer mechanisms were found to be operative in the ranges of water-side Reynolds numbers based on the hydraulic diameter. In a range of Reynolds number from 1,200 to 6,000, the water-side thermal resistance accounts for less than ten percent of the overall thermal resistance. The dominant thermal resistance is always on the air-side. On the other hand, the thermal resistance of water-side is nearly equal to that of air-side in a Reynolds number range from 500 to 1,200.     

System performance of a deep borehole heat exchanger

Deep borehole heat exchanger (BHE) systems, installed in abandoned boreholes, have been operative in Switzerland for several years now. The operational conditions of the 2302 m deep BHE plant at Weggis have been monitored continuously since 1994. In the first operational phase, lasting from October 1994 to May 1996, the plant was severely underused, as shown by the high production temperatures (40 °C). This behaviour was investigated by a numerical model accounting for the heat transport in the rock matrix and along the different tubing systems, with special emphasis on the heat transfer in a multi-layer insulated central pipe. Lacking detailed logging data or undisturbed temperature profiles, an axis-symmetrical model had to be used, assuming uniform rock parameters. Sensitivity studies highlighted the effect of varying flow rate or operation/recovery cycle lengths and helped to develop a strategy that allowed us to make an accurate calculation of the long-term Weggis production history. The initial model assumptions, based on this detailed treatment of the tubing system, could explain the operational data. By means of slight model variations that account only for the minor effects of metallic sleeves, the long-term production temperature history of the Weggis plant could be accurately fitted. These findings were confirmed by a detailed analysis of the May 1996 data. Due to the low degree of utilization, only numerical sensitivity analyses were able to highlight the potential of the deep BHE plant at Weggis. The results indicate that the low utilisation of 40 kW during the first operational phase could be increased to over 200 kW. The specific yield of deep systems is much higher than in conventional shallow BHE systems. Our simulation procedure proves that the heat transfer in a deep BHE system is well understood.     

Analysis of microchannel heat sink performance using nanofluids

In this study, silicon microchannel heat sink performance using nanofluids as coolants was analyzed. The nanofluid was a mixture of pure water and nanoscale Cu particles with various volume fractions. The heat transfer and friction coefficients required in the analysis were based on theoretical models and experimental correlations. In the theoretical model, nanofluid was treated as a single-phase fluid. In the experimental correlation, thermal dispersion due to particle random motion was included. The microchannel heat sink performances for two specific geometries, one with W_ch = W_fin = 100 μm and L_ch = 300 μm, the other with W_ch = W_fin = 57 μm and L_ch = 365 μm, were examined. Because of the increased thermal conductivity and thermal dispersion effects, it was found that the performances were greatly improved for these two specific geometries when nanofluids were used as the coolants. In addition to heat transfer enhancement, the existence of nanoparticles in the fluid did not produce extra pressure drop because of small particle size and low particle volume fraction.     

石墨烯-PFA复合材料换热器与金属换热器的传热性能对比

氟塑料换热器以其耐腐蚀、耐磨损等优点而备受关注,但氟塑料热导率较低,换热能力差,限制了其广泛应用。石墨烯-PFA复合材料兼具石墨烯优异的导热性和可熔性聚四氟乙烯(PFA)良好的耐酸碱腐蚀性,是新一代的换热器材料。搭建了余热回收测试实验台,对石墨烯-PFA复合材料换热器和金属换热器的传热性能进行对比。研究了不同烟气流速、不同进口烟气温度以及不同石墨烯配比对复合材料传热性能的影响。结果表明:对于金属换热器和复合材料换热器,当烟气流速从2.0增加到4.0 m/s时,传热系数分别增加到原来的1.19和1.34倍;随着进口烟温的升高,两种材质的传热系数分别降低了15.6%和14.7%;随着石墨烯含量增加,复合材料的导热系数以及传热系数均增加。     

Comparison of heat pump performance using fin-and-tube and microchannel heat exchangers under frost conditions

Vapor compression heat pumps are drawing more attention in energy saving applications. Microchannel heat exchangers can provide higher performance via less core volume and reduce system refrigerant charge, but little is known about their performance in heat pump systems under frosting conditions. In this study, the system performance of a commercial heat pump using microchannel heat exchangers as evaporator is compared with that using conventional finned-tube heat exchangers numerically and experimentally. The microchannel and finned-tube heat pump system models used for comparison of the microchannel and finned-tube evaporator performance under frosting conditions were developed, considering the effect of maldistribution on both refrigerant and air sides. The quasi-steady-state modeling results are in reasonable agreement with the test data under frost conditions. The refrigerant-side maldistribution is found remarkable impact on the microchannel heat pump system performance under the frost conditions. Parametric study on the fan speed and the fin density under frost conditions are conducted as well to figure out the best trade-off in the design of frost tolerant evaporators.     

Influence of heat exchanger effectiveness on performance of vapour absorption heat transformers

A thermodynamic analysis was made to study the effect of heat exchanger effectiveness on the performance of single stage vapour absorption heat transformers (VAHT). The working fluid pairs considered were R21-DMF and R21-DMETEG. Variations in the performance parameters such as coefficient of performance, exergetic efficiency, concentration difference and circulation ratio at different values of operating temperatures were studied. Among the two working pairs analysed, the R21-DMF pair yielded a high coefficient of performance and exergetic efficiency, whereas the R21-DMETEG pair yielded a high temperature lift at given operating conditions. The improvements in coefficient of performance and exergetic efficiency with heat exchanger effectiveness were more pronounced for R21-DMF than for R21-DMETEG. Correlations are presented for quick estimation of performance under various operating conditions.     

Experimental study on the heat transfer enhancement of MWNT-water nanofluid in a shell and tube heat exchanger

Heat transfer enhancement of multi-walled carbon natube(MWNT)/water nanofluid in a horizontal shell and tube heat exchanger has been studied experimentally. Carbon nanotubes were synthesized by the use of catalytic chemical vapor deposition (CCVD) method over Co–Mo/MgO nanocatalyst. Obtained MWNTs were purified using a three stage method. COOH functional groups were inserted for making the nanotubes hydrophilic and increasing the stability of the nanofluid. The results indicate that heat transfer enhances in the presence of multi-walled nanotubes in comparison with the base fluid.     

Conjugate fluid flow and kinetics modeling for heat exchanger fouling simulation

Thermal treatment of fluid foods represents a major unit operation in the food industry, to ensure the product's safety and quality features. But during the thermal treatments of such sensible fluids in common plate heat exchangers, food constituents such as proteins can be thermally damaged and precipitated to form fouling that greatly affect the treatment efficiency and alter the product's desired features.Computational Fluid Dynamics simulations can then be successfully exploited, bringing forth temperature and velocity information that yield for deposit distributions when coupled to biochemical notations for thermal denaturation of fluid constituents.The present work exploits such modeling for a single-channel heat exchanger during pasteurization of milk. The model enforces a conjugate system of differential equations to a heat exchanger's corrugated plate to combine flow, heat transfer and local transport of β-lactoglobulin. A preliminary computation has been performed that could be applied to geometry optimization (different corrugation shape and orientation) and for a variety of biochemically evolutive products.     

Thermal performance of optimized interrupted microchannel heat sink (IMCHS) using nanofluids

An interrupted microchannel heat sink (IMCHS) using nanofluids as working fluids is analyzed numerically to increase the heat transfer rate. The rectangular IMCHS is designed with length and width of 10 mm and 0.057 mm respectively while optimum cut section number, n_c = 3. The three dimensional governing equations (continuity, momentum and energy) were solved using finite volume method (FVM). Parametric study of thermal performance between pure water-cooled and nanofluid-cooled IMCHS are evaluated for particle diameter in the range of, 30 nm to 60 nm, volume fraction in the range of, 1% to 4%,nanofluid type of Al₂O₃, CuO, and SiO₂ at Reynolds number range of 140 to 1034 are examined. The effects of the transport properties, nanofluid type, nanoparticle volume fraction and particle diameter are investigated on the IMCHS performance. It is inferred that the Nu number for IMCHS is higher than the conventional MCHS with a slight increase of the pressure drop. It is found that highest thermal augmentation is predicted for Al₂O₃, followed by CuO, and finally for SiO₂ in terms of Nu_nf/Nu_pw in the IMCHS. The Nu number increased with the increase of nanoparticle volume fraction and with the decrease of nanoparticle diameter.     

The impact of various nanofluid types on triangular microchannels heat sink cooling performance

This paper discusses the impact of using various types of nanofluids on heat transfer and fluid flow characteristics in triangular shaped microchannel heat sink (MCHS). In this study, an aluminum MCHS performance is examined using water as a base fluid with different types of nanofluids such as Al₂O₃, Ag, CuO, diamond, SiO₂, and TiO₂ as the coolants with nanoparticle volume fraction of 2%. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite volume method. It is inferred that diamond-H₂O nanofluid has the lowest temperature and the highest heat transfer coefficient, while Al₂O₃-H₂O nanofluid has the highest temperature and the lowest heat transfer coefficient. SiO₂-H₂O nanofluid has the highest pressure drop and wall shear stress while Ag-H₂O nanofluid has the lowest pressure drop and wall shear stress among other nanofluid types. Based on the presented results, diamond-H₂O and Ag-H₂O nanofluids are recommended to achieve overall heat transfer enhancement and low pressure drop, respectively, compared with pure water.     

Effects of thermal property variations on the liquid flow and heat transfer in microchannel heat sinks

Three-dimensional conjugate numerical simulations using the inlet, average and variable thermal properties respectively were performed for the laminar water flow and heat transfer in rectangular microchannels with D_h of 0.333 mm at Re of 101–1775. Both average and variable properties are adopted in data reduction. The calculated local and average characteristics of flow and heat transfer are compared among different methods, and with the experiments, correlations and simplified theoretical solution data from published literatures. Compared with the inlet property method, both average and variable property methods have significantly lower f_app, but higher convective heat transfer coefficient h_z and Nu_z. Compared with the average property method, the variable property method has higher f_appRe_ave and lower h_z at the beginning, but lower f_appRe_ave and higher h_z at the later section of the channel. The calculated Nu_ave agree well with the Sieder-Tate correlation and the recently reported experiment, validating the traditional macroscale theory in predicting the flow and heat transfer characteristics in the dimension and Re range of the present work.     

A numerical study of laminar convective heat transfer in microchannel with non-circular cross-section

Three-dimensional numerical simulations of the laminar flow and heat transfer of water in silicon microchannels with non-circular cross-sections (trapezoidal and triangular) were performed. The finite volume method was used to discretize the governing equations. Numerical results were compared with experimental data available in the literature, and good agreements were achieved. The effects of the geometric parameters of the microchannels were investigated, and the variations of Nusselt number with Reynolds number were discussed from the field synergy principle. The simulation results indicate that when the Reynolds numbers are less than 100, the synergy between velocity and temperature gradient is much better than the case with Reynolds number larger than 100. There is an abrupt change in the intersection angle between velocity and temperature gradient around Re=100. In the low Reynolds number region the Nusselt number is almost proportional to the Reynolds number, while in the high Reynolds number region, the increasing trend of Nusselt number with Reynolds number is much more mildly, which showed the applicability of the field synergy principle. In addition, for the cases studied the fully developed Nusselt number for the microchannels simulated increases with the increasing Reynolds number, rather than a constant.     

微通道内流动沸腾特性研究

对国内外微通道流动和换热的研究实验作了总结,阐述了影响微通道换热系数的因素,如热流密度、过热度和干度等.对去离子水在内径为0.65 mm、长为102 mm的圆形管道内流动沸腾换热进行了实验研究,得到了局部换热系数随干度的变化关系,进而根据换热系数的变化趋势讨论了饱和流动沸腾区微通道内主导的换热机制.结果表明:从换热系数随干度的变化关系很难判定主导的换热机制;将实验数据与已发表的预测关联式进行了比较,发现大多关联式都失效,说明基于常规理论的模型不再适用于微通道.