• Energy Research

Highly conductive inserts improve heat removal from electronic components remarkably; however, using these inserts is associated with a side effect: the mechanical strength deterioration of the piece. In this research, based on the constructal theory, the architecture of these inserts is optimized in such a way that the lowest maximum temperature and highest mechanical strength of the disc-shaped piece are achieved. Here, three patterns for the configuration of inserts embedded in the disc are studied, namely radial, dendritic with one level of pairing and dendritic with two levels of pairing. The total amount of high-conductivity inserts material is fixed, while the geometrical parameters of the inserts like length, the bifurcating locations and the angles of bifurcations are considered as degrees of freedom. It was observed that when 16 inserts were invoked, maximum temperature and stress were minimized. Results showed that changing the structure from radial to dendritic with one bifurcation causes a decrease in the maximum temperature and stress of about 17% and 3.2%, respectively. Also, it was seen that increasing the complexity of the structure by increasing the number of bifurcations from one to two leads to 5.6% reduction and 1% increment in minimal maximum temperature and optimal maximum stress, respectively.

In the present study, a thermodynamic second-law analysis was performed to investigate the effects of different geometry and flow parameters on the air-cooled heat exchanger performance. For this purpose, the entropy generation due to heat transfer and pressure loss of internal and external flows of the air-cooled heat exchanger was calculated; and it was observed that the total entropy generation has a minimum at special tube-side Reynolds number. Also, it was seen that the increasing of the tube-side Reynolds number resulted in the rise of the irreversibility of the air-cooled heat exchanger. The results also showed when air-side Reynolds number decreased, the entropy generation rate of the external flow reduced. Finally, based on the computed results, a new correlation was developed to predict the optimum Reynolds number of the tube-side fluid flow.

In the present study, flow and forced convective heat transfer in an air heater conduit filled with a porous matrix with a uniform constant solar heat flux has been investigated analytically, based on minimal entropy generation principle. While trying to decrease entropy generation due to heat transfer, pressure loss entropy generation increases, which indicates that an optimal porosity value exists. The influence of Reynolds number, fluid properties, constant uniform heat flux, flow, and geometry of the system on the optimum matrix porosity has been investigated. It was revealed that optimum matrix porosity values increase as Reynolds number increases. In the range of the present study, a correlation predicting optimal matrix porosity was proposed using least squares analysis.

In this research, we consider the generation of conductive heat trees at microscales and nanoscales for cooling electronics which are considered as heat-generating disk-shaped solids. With the advent of nanotechnology and the production of electronics in micro- and nanoscales in recent years, designing workable systems for cooling them is considered widely. Therefore, tree-shape conduction paths of highly conductive material including radial patterns, structures with one level of branching, tree-with-loop architectures, and combination of structures with branching and structures with loops are generated for cooling such electronic devices. Furthermore, constructal method which is used to analytically generate heat trees for cooling a disk-shaped body is modified in the present work, that we call it modified analytical method. Moreover, every feature of the tree architectures is optimized numerically to make a comparison between numerical and analytical results and to generate novel architectures. Since there are some constructal tree architectures which are not possible to be generated analytically, numerical approach is used for optimization. When the smallest features of the internal structure are smaller than mean free path of the energy carriers, heat conductivity is no longer a constant and becomes a function of the smallest dimension of the structure. Therefore, we consider models which were proposed for estimating conductivity of small scale bodies.

Abstract In the present study, two-phase refrigerant flow is simulated using drift flux model for straight and helical capillary tubes. The conservation equations of mass, energy and momentum are solved using the 4th order Runge–Kutta method. This model is validated by previously published experimental and numerical results and also by experimental results presented in this work. The effect of various parameters such as inlet pressure, inlet temperature, sub-cooling degree, and geometric dimensions are studied. The results of the present study show that for the same length and under similar conditions, mass flux through helical capillary tube with coil diameter of 40 mm are about 11% less than that through the straight tube, where the helical tube length is about 14% shorter than the straight one for the same refrigerant mass flux.

In this study, an innovative system is introduced to cool down the electronic components in the case of a malfunction of the equipment. A rectangular piece is considered in which heat is generated, while parallel channels are embedded into it to cool the element. In some of these channels (active channels), the cooling fluid flows, while in others (passive channels) fluid is stationary. The active and passive channels are placed alternately and separated by micro-thermostats. When malfunction happens, the heat flux is increased in an arbitrary spot of the piece, opening the closest thermostat and turning the adjacent passive channel into active channel. Therefore, the coolant flow at that portion is increased which dissipates the surcharge heat. The fluid flow is steady and laminar. The thermal entry length is considered, and the effect of local pressure loss is studied. The governing equations are resolved, analytically. Five different cases are studied. At three cases, one hot spot occurs at different locations; while at the other two cases, two hot spots happen simultaneously over the electronic board. Results show that invoking the recommended system at a malfunction situation can reduce the maximum temperature of the electronic piece up to 13.3, 15.2, 17.2, 19.3 and 17.0 °C for cases I–V, respectively; for hot spot heat flux 40 kW m−2.

One of the cooling methods in engineering systems is usage of phase change materials. Phase change materials or PCMs, which have high latent heats, are usually used where high energy absorption in a constant temperature is required. This work presents a numerical analysis of PCMs effects on cooling Li-ion batteries and their decrease in temperature levels during intense discharge. In this study, three PCM shells with different thermo-physical specifications located around a battery pack is examined. The results of each possible arrangement are compared together and the best arrangement leading to the lowest battery temperature during discharge is identified. In addition, the recovery time for the system which is the time required for the PCMs to refreeze is investigated.

The present paper examines the optimization of triangular microchannel heat sinks. The impact of volume fraction of solid material and pressure drop on the maximum temperature of the microchannel heat sinks are investigated and their optimum operating conditions are compared. From the results, it is seen that increasing the side angle of the triangular microchannel, improves its performance. Furthermore, there is an appropriate agreement between the analytical and numerical results. Finally, the effect of degrees of freedom on the performance of microchannels is investigated. To accomplish this end, the triangular microchannels with the side angle of 60 degree have been chosen as it has the best performance compared to other microchannels. It is observed that the minimized maximum temperatures of optimized microchannel heat sinks with three degrees of freedom are 10% lower than the ones with two degrees of freedom.

Abstract Maintaining the peak temperature of a heat source under an allowable level has always been a major concern for engineers engaged in the design of cooling systems for electronic equipment. The primary goal of this paper is to examine the advantages and/or disadvantages of placing a conductive thick plate as a heat transfer interface between a heat source and a cold flowing fluid. In such arrangement, the heat source is cooled under the thick plate instead of being cooled in direct contact with the cooling fluid. It is demonstrated that the thick plate can significantly improve the heat transfer between the heat source and the cooling fluid by way of conducting the heat current in an optimal manner. The two most attractive advantages of this method are that no additional pumping power and no extra heat transfer surface area, that is quite different from fins (extended surfaces). Unlike related archival papers in the literature, the present paper allows open spaces toward optimization. The objective is to minimize the maximum temperature, the ‘hot spot’. Detailed analytical expressions are presented and a numerical analysis is carried out on the conservation equations based on the SIMPLEC algorithm. It is categorically proved that there exists an optimal thickness of the thick plate, which minimizes the peak temperature. Also, it is shown that the efficiency of the optimized plate on minimizing the target peak temperature depends upon the Reynolds number of the fluid flow and the material thermal conductivity.

Abstract In this article, the economic performance of the solar-driven steam ejector refrigeration system is addressed due to the rising beneficial adoption of this system with regard to using low-grade heat sources. For this purpose, a detailed exergoeconomic analysis on four proposed configurations of this system integrated with either or both of a precooler and a preheater is performed. At a refrigeration load of 5 kW, the results of this analysis show that it is more advantageous for all configurations to have evaporators work at 278 K with condensers remain at 311 K, whilst the generators are kept working at 362 K. The characteristics of each configuration are comprehensively discussed and compared. Based on the assessment of economic parameters, the fourth configuration provides the highest exergetic efficiency and the lowest total investment cost (0.1891 $/h) compared to the other configurations. The current work furnishes essential information that paves the way for future exergoeconomic studies in this field.