Computation of the flow and thermal fields in a thermoacoustic refrigerator

The hydro- and thermodynamic processes near and within two-dimensional stack plates are simulated by numerical solution of the unsteady compressible Navier–Stokes, continuity, energy equations, and the equation of state (for air as the working fluid). The stack is assumed to consist of flat plates of equal thickness. The second order mean velocity field is computed in the neighborhood of the stack plates. In the stack plate extremities the vortical mean flow is observed which is due to the abrupt change of a slip condition to a no-slip velocity boundary condition. The temperature of the stack is governed by the energy equation; therefore the entire problem is treated as a conjugate heat transfer problem. The temperature fields in the neighborhood of the solid stack plate are also observed. From the location of the heat exchangers in Fig. 1(a), it is obvious that knowledge of the flow and thermal fields at the edges of the stack plates is the key for the development of a systematic design methodology for heat exchangers in thermoacoustic devices.     

Analytical and empirical determination of thermal performance of louvered heat exchanger – Effects of air flow statistics

Using the basic equations for modeling heat transfer through heat exchangers, an analytical approach is developed to determining the thermal performance of cross-flow air-cooled heat exchangers as a function of the flow statistics of the upstream cooling air. A two-dimensional computational code is also developed to calculate heat-exchanger performance in relation to the airflow topology upstream of the heat exchanger induced by its integration in complex environments such as the car underhood compartment. The analytical and numerical results show satisfactory agreement: the mean relative error in heat exchanger thermal performance determined by the numerical computation and the analytical approach is about 0.5%.     

Augmentation of Heat Transfer by Creation of Streamwise Longitudinal Vortices Using Vortex Generators

This paper summarizes the current state of the art related to improvement of the heat exchanger surfaces using streamwise longitudinal vortices. Primarily, the improvements related to fin-tube cross-flow heat exchangers and the plate-fin heat exchangers have been addressed. Protrusions in certain forms, such as delta wings or winglet pairs, act as vortex generators, which can enhance the rate of heat transfer from the heat-exchanger surfaces that may be flat or louvered. The strategically placed vortex generators create longitudinal vortices, which disrupt the growth of the thermal boundary layer, promote mixing between fluid layers, and hence lead to augmentation in heat transfer. The flow fields are dominated by swirling motion associated with modest pressure penalty. Heat transfer is augmented substantially for all the proposed configurations of the longitudinal vortex generators, such as delta wings, rectangular winglet pairs, and delta winglet pairs, with varying degree of pressure penalty. Both computational and experimental investigations on flow and heat transfer in the heat exchanger passages with built-in vortex generators are revisited and summarized.     

自然循环在冲击热负荷作用下的流动特性实验研究

冲击热负荷条件下的自然循环是工程中常见的汽液两相流动与传热过程 ,同时也是一个复杂的非稳态过程。转炉余热锅炉是一种典型的运行于冲击热负荷下的大型换热设备 ,冲击热负荷严重地影响着其水循环流动特性 ,从而影响它的寿命。在实验模拟氧气顶吹炼钢转炉余热锅炉的实际工作过程的基础上 ,研究了冲击热负荷、压力和初始速度等因素对不稳定传热和流动的影响。实验结果表明 ,自然循环在受到冲击热负荷后 ,水循环流量迅速达到最大值 ,呈现较快的响应速度 ,随后在最大流量附近脉动。循环流量的主要影响因素是冲击热负荷的强度 ,而压力和初始速度对循环流量的影响则较小。     

Numerical computation for parallel plate thermoacoustic heat exchangers in standing wave oscillatory flow

A simplified computational method for studying the heat transfer characteristics of parallel plate thermoacoustic heat exchangers is presented. The model integrates the thermoacoustic equations of the standard linear theory into an energy balance-based numerical calculus scheme. Details of the time-averaged temperature and heat flux density distributions within a representative domain of the heat exchangers and adjoining stack are given. The effect of operation conditions and geometrical parameters on the heat exchanger performance is investigated and main conclusions relevant for HX design are drawn as far as fin length, fin spacing, blockage ratio, gas and secondary fluid-side heat transfer coefficients are concerned. Most relevant is that the fin length and spacing affect in conjunction the heat exchanger behavior and have to be simultaneously optimized to minimize thermal losses localized at the HX-stack junctions. Model predictions fit experimental data found in literature within 36% and 49% respectively at moderate and high acoustic Reynolds numbers.     

Spatial optimization of an underhood cooling module – Towards an innovative control approach

The present paper reports a numerical investigation of spatial optimization of heat-exchanger by acting on its positioning in the vehicle’s cooling module. This analysis also elucidates how to act on the different parameters influencing heat-exchanger performance in order to optimize their functioning. A two-dimensional computation code permits optimizing the performance of the cooling module by positioning different heat exchangers, in both the driving and stop phases of the vehicle. The ultimate aim is to apply new control approaches to real vehicles so as to reduce pump and compressor energy consumption and thus fuel consumption. Compared to a reference “in-series” configuration of the cooling module HXs (in which the different HXs are superposed in the airflow direction), an “in-parallel” configuration (in which the different HX surfaces are in a row with respect to the air flow direction) increases the thermal power of the HXs by 4.4% and decreases the pressure losses by 0.9%.     

Numerical study of heat pipe application in heat recovery systems

Heat pipes are two-phase heat transfer devices with extremely high effective thermal conductivity. They can be cylindrical or planar in structure. Heat pipes can be embedded in a metal cooling plate, which is attached to the heat source, and can also be assembled with a fin stack for fluid heat transfer. Due to the high heat transport capacity, heat exchangers with heat pipes have become much smaller than traditional heat exchangers in handling high heat fluxes. With the working fluid in a heat pipe, heat can be absorbed on the evaporator region and transported to the condenser region where the vapour condenses releasing the heat to the cooling media. Heat pipe technology has found increasing applications in enhancing the thermal performance of heat exchangers in microelectronics, energy and other industrial sectors.Utilisation of a heat pipe fin stack in the drying cycle of domestic appliances for heat recovery may lead to a significant energy saving in the domestic sector. However, the design of the heat pipe heat exchanger will meet a number of challenges. This paper presents a design method by using CFD simulation of the dehumidification process with heat pipe heat exchangers. The strategies of simulating the process with heat pipes are presented. The calculated results show that the method can be further used to optimise the design of the heat pipe fin stack. The study suggests that CFD modelling is able to predict thermal performance of the dehumidification solution with heat pipe heat exchangers.     

Power optimization of an endoreversible closed intercooled regenerated Brayton-cycle coupled to variable-temperature heat-reservoirs

In this paper, in the viewpoint of finite-time thermodynamics and entropy-generation minimization are employed. The analytical formulae relating the power and pressure-ratio are derived assuming heat-resistance losses in the four heat-exchangers (hot- and cold-side heat exchangers, the intercooler and the regenerator), and the effect of the finite thermal-capacity rate of the heat reservoirs. The power optimization is performed by searching the optimum heat-conductance distributions among the four heat-exchangers for a fixed total heat-exchanger inventory, and by searching for the optimum intercooling pressure-ratio. When the optimization is performed with respect to the total pressure-ratio of the cycle, the maximum power is maximized twice and a ‘double-maximum’ power is obtained. When the optimization is performed with respect to the thermal capacitance rate ratio between the working fluid and the heat reservoir, the double-maximum power is maximized again and a thrice-maximum power is obtained. The effects of the heat reservoir’s inlet-temperature ratio and the total heat-exchanger inventory on the optimal performance of the cycle are analyzed by numerical examples.     

Experimental study on the heat transfer at the heat exchanger of the thermoacoustic refrigerating system

Oscillatory flow heat transfer at the heat exchanger of the thermoacoustic refrigeration system was studied. The study identified significant factors that influence this heat transfer as well as the construction of the system. The results from the experimental study were correlated in terms of Nusselt number, Prandtl number and Reynolds number to obtain a useful new correlation for the heat transfer at the heat exchangers. Results show that using straight flow heat transfer correlations for analyses and design of this system could result in significant errors. Results also show the relationship between the oscillatory heat transfer coefficient at the heat exchangers, the mean pressure and frequency of oscillation. Higher mean pressures result in greater heat transfer coefficients if the thermoacoustic refrigerating system operates at the corresponding resonant frequency. However, a compromise has to be reached to accommodate construction of the stack.     

Methane steam reforming in a novel ceramic microchannel reactor

Microchannel heat exchangers and reactors can deliver very high performance in small packages. Such heat exchangers are typically fabricated from aluminum, copper, stainless steel, and silicon materials. Ceramic microchannel reactors offer some significant advantages over their metallic counterparts, including very-high-temperature operation, corrosion resistance in harsh chemical environments, low cost of materials and manufacturing, and compatibility with ceramic-supported catalysts. This work describes a ceramic microchannel reactor that achieves process intensification by combining heat-exchanger and catalytic-reactor functions to produce syngas. A complete computational fluid dynamics (CFD) model as well as a geometrically simplified hybrid CFD/chemical kinetics model is used in conjunction with experimentation to examine heat transfer, fluid flow, and chemical kinetics within the ceramic microchannel structure. Heat-exchanger effectiveness of up to 88% is experimentally demonstrated. Reactive heat-exchanger performance for methane-steam reforming reaches 100% methane conversion and high selectivity to syngas at a gas hourly space velocities (GHSV) of 15,000 h⁻¹. Model results agree well with experimental data and provide insight into physical processes underway during reactor operation.     

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.     

Performance Comparison of Rectangular and Air-Foil Shapes Offset Strip Flow Paths Micro-Heat Exchangers

This study designed and tested eight micro-heat exchangers with rectangle-, air-foil-, and shuttle-type strips for use in the liquid cooling system. The effects of strip length, strip type, and strip arrangement were considered for heat transfer performance comparison. The test results show that the heat exchangers with shorter strip length and narrower strip space provide better heat transfer performance. The short air-foil strips heat exchanger with 1.0 mm strip length performed the lowest thermal resistance among all types of heat exchangers. Because of its narrow flow paths, the performance of the overlapped shuttle strip heat exchanger is better than that of the offset shuttle, long air-foil, and rectangle strip heat exchangers. However, the space between strips is limited by the fabrication techniques and is difficult to be made narrower by the method of chemical etching.     

Optimising heat exchangers for air-to-air space-heating heat pumps in the United Kingdom

The paper deals with some of the major aspects of heat-exchanger design for electric heat pumps. After a discussion of heat-transfer theory, it describes a method that can be used in the design and sizing of air-to-refrigerant heat exchangers and in calculating temperature distributions. As an illustration, economically optimum sizes for exchanger coils are given for heat pumps of output 13.9 kW, 5.6 kW and 5 kW at 5°C outside the ambient temperature. At several stages, manufacturer's experimental data have been used, and the final results are compared with the design of heat exchangers used in commercially available models. Some temperature measurements made on a heat-pump installation in an experimental house are also reported. At least doubling the size of presently used indoor coils is shown to be economically justifiable, increasing the seasonal coefficient of performance from about 2.4 to 2.8-3.0. Reassessment of outdoor-fan size is also shown to be necessary. Throughout the work it is assumed that the heat pump is required for heating only, as would be the case in the United Kingdom.     

Compact Air-to-Air Thermosyphon Heat Exchanger

Outdoor cabinets containing power electronics components need to be cooled effectively and at the same time protected from outside air, which may contain moisture and various kinds of dirt that would reduce the reliability of the electronics. Air-to-air heat exchangers are widely used in the industry as they are cost-competitive and easy to install and maintain. On the other hand, they are inefficient and bulky. ABB holds a patent on a cost-effective modular compact thermosyphon-based air-to-air heat exchanger for power electronics cabinets. This technology uses numerous multiport extruded tubes with capillary-sized channels disposed in parallel to achieve the desired compactness. The heat exchanger is made of a stack of thermosyphon units to cope with the required heat loads. The experimental performance of this novel power electronics cooling system with R134a was measured for a single unit and a stack of thermosyphons. The influence of different parameters such as the heat load, fluid filling ratio, air temperature, and flow rate were investigated. A numerical model was developed in order to predict the performance of the thermosyphon unit and stack for various and changing operating conditions. Prediction shows good agreement with the experimental results.     

Second Law Efficiency Analysis of Heat Exchangers

Analytical analysis of unbalanced heat exchangers is carried out to study the second law thermodynamic performance parameter through second law efficiency by varying length‐to‐diameter ratio for counter flow and parallel flow configurations. In a single closed form expression, three important irreversibilities occurring in the heat exchangers—namely, due to heat transfer, pressure drop, and imbalance between the mass flow streams—are considered, which is not possible in first law thermodynamic analysis. The study is carried out by giving special influence to geometric characteristics like tube length‐to‐diameter dimensions; working conditions like changing heat capacity ratio, changing the value of maximum heat capacity rate on the hot stream and cold stream separately and fluid flow type, i.e., laminar and turbulent flows for a fully developed condition. Further, second law efficiency analysis is carried out for condenser and evaporator heat exchangers by varying the effectiveness and number of heat transfer units for different values of inlet temperature to reference the temperature ratio by considering heat transfer irreversibility. Optimum heat exchanger geometrical dimensions, namely length‐to‐diameter ratio can be obtained from the second law analysis corresponding to lower total entropy generation and higher second law efficiency. Second law analysis incorporates all the heat exchanger irreversibilities. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21109     

NUMERICAL ANALYSIS OF FLUID FLOW AND HEAT TRANSFER CHARACTERISTICS IN THREE-DIMENSIONAL PLATE FIN-AND-TUBE HEAT EXCHANGERS

This numerical study provides three-dimensional (3-D) time-dependent modeling of unsteady laminar flow and heat transfer over single- and multirow plate fin-and-tube heat exchangers. The complex nature of the flow field featuring a horseshoe vortex is investigated for both configurations. The time-dependent evolution of the horseshoe vortex mechanism on the forward part of the tube and its journey to the rear of the tube are studied to provide fundamental information on the local flow structure and the corresponding heat transfer characteristics. The effects of various governing parameters, such as fin spacing, Reynolds number, tube row number, and tube arrangement, on the heat transfer and flow characteristics are also studied for the Reynolds number range investigated. It is found that the local flow structure including formation and evolution of vortex systems and singular-point interactions correlates strongly with the heat transfer characteristics. The numerical results for the integral heat transfer parameters agree well with available experimental measurements.     

Power-based performance comparison between carbon dioxide and R125 transcritical cycles for a low-grade heat source

In order to compare the power output of the carbon dioxide transcritical cycle and the R125 transcritical cycle for a low-grade heat source of about 100 °C, the two cycles were optimized for power output using a simulation method. In contrast to conventional approaches, each working fluid’s heat transfer and pressure drop characteristics within the heat exchangers were taken into account by using a discretized heat exchanger model. To fairly compare the power output of the cycles by using different working fluids, the inlet temperatures and the flow rates of both the heat source and the heat sink were fixed. The cycle minimum temperature was not given, but was determined by the heat sink conditions and the working fluid’s heat transfer and pressure drop characteristics, as it is in actual practice. The total heat transfer area was fixed, whereas the allocation of the heat-exchanger area between the vapor generator and the condenser was optimized in the simulation. The R125 transcritical cycle produced 14% more power than did the carbon dioxide transcritical cycle. Even though the carbon dioxide cycle shows better heat transfer and pressure drop characteristics in the heat exchangers, the high pumping power required to manage the large pressure head degrades the cycle’s power output. Based on this study, the R125 transcritical cycle is recommended for heat sources of about 100 °C.     

Effects of flow direction and thermal short-circuiting on the performance of small coaxial ground heat exchangers

The effects of flow direction and thermal short-circuiting on the performance of small-size coaxial ground heat exchangers, currently used in Northern Italy, are studied by finite-element simulations, performed through the software package COMSOL Multiphysics 3.4 (©Comsol, Inc.). The real 2-D axisymmetric unsteady heat conduction and convection problem is considered, both for winter and for summer working conditions. The flow in the outer annular passage is laminar in winter and turbulent in summer. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the weak form boundary condition available in COMSOL Multiphysics; the forced-convection heat transfer in the outer annular passage is simulated directly. Two Small Coaxial Ground Heat Exchangers (SCGHEs) with the same length (20 m) but different cross-sections are examined; moreover, two values of the ground thermal conductivity, as well as two materials for the inner tube wall are considered. The results point out that the annulus-in flow direction (fluid inlet in the outer annular passage) is more efficient than the center-in flow direction (fluid inlet in the inner circular tube) and that, on account of the small length, the effect of thermal short-circuiting is not important for SCGHEs, especially if the annulus-in flow direction is employed.     

板翅式换热器翅片及隔板动态特性分析

基于板翅式换热器翅片的非稳态导热方程，计算分析了翅片的动态特性，认为翅片的不稳定传热过程相对于换热器其它过程特征时间无限小，因此可以不考虑翅片的动态特性，从而简化了板翅式换热器动态模型，通过分析换热器动态过渡过程表明：隔板的热容对板翅式换热器的动态特性的影响是不容忽略的。     

Improving the thermal performance of coaxial borehole heat exchangers

Borehole heat exchangers are the fundamental component of ground coupled heat pumps, which are now widely employed for energy saving in building heating and cooling. The improvement of the thermal efficiency of Coaxial Borehole Heat Exchangers (CBHEs) is pursued in this paper by investigating the effects of thermal short-circuiting and of flow rate, as well as of the constituent materials and of the geometrical configuration of the CBHE cross section. The analysis is performed by means of finite-element simulations, implemented through the software package COMSOL Multiphysics. The real 2-D axisymmetric unsteady heat transfer problem is modelled, for both winter and summer working conditions, by considering CBHEs with a length of 100 m placed either in a high conductivity or in a low conductivity ground. The results point out that the effects of flow rate and of thermal short-circuiting are both important, and that the latter can be reduced considerably by employing a low conductivity material, such as PPR80, for the inner tube. Finally, it is shown that the performance of the CBHE could be improved, with respect to the commonly used geometry, by increasing the diameter of the inner tube while leaving the outer tube unchanged.