Minimum Thermal Resistance Principle Research Articles

Variabilities and their upper and lower bounds of the equivalent thermal conductivity and resistance defined by the entransy dissipation rate

The prediction of equivalent thermal conductivity (ETC) and resistance (ETR) is an important topic in the composite material field. Many expressions of ETCs and ETRs have been proposed using different homogenization methods in last several decades. Especially, expressions defined by the entransy dissipation rate, keff and Reff, excel for their wide range of application. Usually, it was agreed that keff and Reff were determined by the thermal conductivity distribution inside. However, we find that they are also vary with the boundary temperature gradient distribution (TGD). In this paper, we simulate and mathematically prove the influence of boundary TGD on keff. The results indicate that keff with uniform boundary TGD is higher than that with non-uniform boundary TGD. In addition, the mathematical proof after a little adjustment is applied to confirm the minimum thermal resistance principle based on the uniform TGD inside. The results show that keff is the maximum when TGD are uniform in all regions including boundaries, and keff reaches the minimum when the non-uniformity of TGD extends to the limit. Upper and lower bounds of keff and Reff found in this paper are greatly valuable for optimizing the heat transfer ability and heat insulation ability of materials. And the methodology as well as conclusion in this paper can be reproduced in some generalized irreversible transport systems.

Investigation of a rectangular heat pipe radiator with parallel heat flow structure for cooling high-power IGBT modules

The thermal management of insulated-gate bipolar transistor (IGBT) modules is a critical issue in the field of power electronics. According to the minimum thermal resistance principle, this study proposed a rectangular heat pipe radiator with parallel heat flow structure that can be used for cooling two 1700 V/1000 A IGBT modules. The prototypes of typical and novel heat pipe radiators were produced to validate the heat transfer enhancement of the novel structure. Performance evaluation and analysis were numerically and experimentally carried out, and the numerical results agreed well with the experimental data. Parametric studies were performed to analyze the effects of inlet air temperature, air volume, and heat load on the heat dissipation capability. The study found that the novel heat pipe radiator has a good start-up characteristic due to the parallel heat flow structure. Moreover, the performance enhancement of the novel heat pipe radiator is obvious at large heat loads. Experiment results show that the two IGBT modules with 3500 W could be cooled down to 67.8 °C when the air volume is 450 m3 h−1, which is 8.9% lower than that of a typical heat pipe radiator. The proposed novel structure improves thermal performance of the novel heat pipe radiator by significantly decreasing its thermal resistance by 22% in comparison with that of a typical heat pipe radiator.

Multi-parameter optimization of shell-and-tube heat exchanger with helical baffles based on entransy theory

An optimization strategy combined the entransy theory and genetic algorithm was proposed in this paper. The applicability of entransy theory on shell-and-tube heat exchanger with helical baffles (STHXsHB) was firstly proved by the consistency of performance evaluation criteria (PEC) and entransy dissipation theory based on experimental data. The effects of helix angle, overlapped degree and shell-side velocity on thermal resistance (optimization objective) were discussed based on the minimum thermal resistance principle. The results showed that the entransy dissipation thermal resistance decreased with the shell-side velocity but increased with the helix angle. The overlapped degree nearly had no effect on the variation of entransy dissipation thermal resistance. According to the sensitivity study, the thermal resistance was considered to be mostly affected by the shell-side velocity and then the helix angle. Three optimum configurations were obtained by the genetic algorithm, for which the thermal resistance decreased by 7%. The combination of entransy theory and genetic algorithm can provide some theoretical guidance to optimize the configuration of shell-and-tube heat exchanger with helical baffles effectively.

最小热阻原理在组合式相变材料蓄热过程优化中的应用

The minimum thermal resistance principle has been developed in recent years, which represents a new method for optimization of heat transfer processes. In this paper, the minimum thermal resistance principle is further developed for optimization of melting process with multiple phase change materials (PCMs). The general formulas of the optimum melting temperature are deduced on the principle of calculus variation, subject to different optimization objectives. Base on the general formulas, the effects of numbers of PCMs on entransy dissipation-based thermal resistance and thermal performance are discussed. The more PCMs used in melting process can reduce the entransy dissipation-based thermal resistance to improve the thermal performance. The results show that the minimum thermal resistance is suitable for melting process optimization with multiple PCMs. Moreover, the combinational efficiency is defined to estimate the thermal performance for multiple PCMs conveniently. The effects of the number of heat transfer unit and heat capacity flowrate on entransy dissipation-based thermal resistance are investigated. The results indicate that, the combinational efficiency can be increased with increasing of numbers of PCMs, and the entransy dissipation-based thermal resistance can be decreased with increasing of the number of heat transfer unit and heat capacity flowrate, and both of the effects are gradually weakened. Thus, a theoretical guidance is provided for selecting the multiple PCMs and designing flow and structure parameters. In addition, based on the minimum thermal resistance principle, the uniformity principle of temperature difference field is proposed as a new optimization criterion for melting process with multiple PCMs.

A thermal environmental analysis method for data centers

The rapidly growing of data center energy consumption rates are contributing to concern about energy conservation. Reasonable airflow organization is an effective way to reduce the cooling system energy consumption and to upgrade the energy utilization efficiency. Comprehensive and accurate analyses are important for airflow organization optimization. This paper presents the entransy dissipation analysis method to analyze the influences of hot and cold air mixing and cooling air distributions on data center heat transfer process. The thermal resistance based on entransy dissipation is used to define three indexes to evaluate hot and cold air mixing, cooling air distributions and the integrated heat transfer performance. A sample data center is studied with the analytical results showing that the smaller the entransy-dissipation-based thermal resistance is, the lower the average heat source temperature will be. Thus, the minimum thermal resistance principle is applicable to the analysis and optimization of data center heat transfer process. The three indexes can be used to guide the improvement and optimization of airflow organization.

Application of entransy in the analysis of HVAC systems in buildings

The main task of HVAC systems in the cooling condition is to remove heat from indoor environment to outdoor environment. HVAC systems are complex networks of various processes, e.g., heat transfer, heat–work conversion, heat–humidity conversion, etc. These processes correspond to equipment such as heat exchangers, indoor cooling terminals, heat pumps, and cooling towers. Single analysis method or thermal parameter could hardly describe all the processes in an HVAC system. Exergy destruction refers to the loss of heat–work conversion ability. Reducing exergy destruction indicates less supplied exergy (input work) of HVAC systems. Entransy is a new parameter defined as heat transfer ability. Entransy dissipation refers to the loss of heat transfer ability. When the purpose of heat transfer is cooling or heating, entransy analysis is a direct method for optimizing heat transfer processes. Loss of HVAC system is mainly in heat transfer process. The entransy dissipation extremum principle or the minimum thermal resistance principle is suitable for analyzing heat transfer process in HVAC system. For indoor cooling, reducing entransy dissipation will increase chilled water temperature. Flow unmatched coefficient ξ represents an increase of thermal resistance of heat exchanger if the calorific capacities of the fluids are different.

Entransy dissipation analysis and optimization of separated heat pipe system

Seperated heat pipe systems are widely used in the fields of waste heat recovery and air conditioning due to their high heat transfer capability, and optimization of heat transfer process plays an important role in high-efficiency energy utilization and energy conservation. In this paper, the entransy dissipation analysis is conducted for the separated heat pipe system, and the result indicates that minimum thermal resistance principle is applicable to the optimization of the separated heat pipe system. Whether in the applications of waste heat recovery or air conditioning, the smaller the entransy-dissipation-based thermal resistance of the separated heat pipe system is, the better the heat transfer performance will be. Based on the minimum thermal resistance principle, the optimal area allocation relationship between evaporator and condenser is deduced, which is numerically verified in the optimation design of separated heat pipe system.

Entransy-dissipation-based thermal resistance analysis of heat exchanger networks

Heat exchanger network optimization has an important role in high-efficiency energy utilization and energy conservation. The thermal resistance of a heat exchanger network is defined based on its entransy dissipation. In two-stream heat exchanger networks, only heat exchanges between hot and cold fluids are considered. Thermal resistance analysis indicates that the maximum heat transfer rate between two fluids corresponds to the minimum entransy-dissipation-based thermal resistance; i.e. the minimum thermal resistance principle can be exploited in optimizing heat exchanger networks.

Entransy flux of thermal radiation and its application to enclosures with opaque surfaces

Entransy is a new concept developed in recent years to measure the transport ability of heat at a temperature in conduction and convection. This paper develops the concept of entransy flux for thermal radiation in enclosures with opaque surfaces. The entransy balance equation and entransy dissipation function are derived. The minimum principle of radiative entransy loss is developed. The potentials and the heat fluxes distribution which meet the Stefan–Boltzmann’s law and the energy balance equation would make the radiative entransy loss minimum if the net heat flux of each surface or the thermal potentials of the surfaces are given. The extremum entransy dissipation principles (EEDP) for thermal radiation are developed. The minimum radiative entransy dissipation leads to the minimum average radiative thermal potential difference for prescribed total heat exchange and the maximum radiative entransy dissipation leads to the maximum heat exchange for prescribed average radiative thermal potential difference. The minimum and maximum principle can be concluded into the minimum thermal resistance principle (MTRP) for thermal radiation by defining the thermal resistance with the entransy dissipation. The EEDP or MTRP is proved to be reliable when they are used to optimize some radiative heat transfer problems, and a comparison is made between the minimum principle of entropy generation and the EEDP.

An equation of entransy transfer and its application

Entransy is a physical quantity describing heat transfer ability, and heat transfer is accompanied by entransy transfer. Thermal energy is conserved in its transfer process, while entransy is dissipated because of the irreversibility of its transfer process. As a result, entransy transfer must have its rules which are different from those of thermal energy transfer. Based on the definition of entransy, an entransy transfer equation is derived, which describes the entransy transfer processes of a multi-component viscous fluid subject to heat transfer by conduction and convection, mass diffusion and chemical reactions. The expressions of entransy flux and entransy dissipation are obtained simultaneously, and their physical mechanism is clarified. And further, the theory and method of optimizing heat transfer applying the entransy transfer equation to the steady-state convection heat transfer process are expounded. The minimum thermal resistance principle and the entransy dissipation extremum principle are obtained by applying the steady-state entransy transfer equation to the steady-state convection heat transfer process. The cases of the single-component steady-state convection heat transfer and the steady-state heat conduction show the application of the theory and method.

Optimization for a heat exchanger couple based on the minimum thermal resistance principle

Following the brief introduction to the concept of a physical quantity, entransy, the equivalent thermal resistance of a heat exchanger couple is defined based on the entransy dissipation. The minimum thermal resistance principle is applied to obtain the optimal heat capacity rate of the medium fluid and the optimal allocation of heat exchangers thermal conductance, which correspond to the maximum heat transfer rate in the heat exchanger couple. In addition, analytical expression for the optimal heat capacity rate of the medium fluid is derived, whose reciprocal equals the sum of the reciprocal of the individual heat capacity rate of the hot and cold fluids, just like the case of two electrical capacitors in series. Numerical results in the variation of the thermal resistance and the heat transfer rate with the medium fluid heat capacity rate or the thermal conductance allocation agree with the theoretical analyses. Finally, for comparison, the entropy generation rate is also calculated to obtain its relation with the thermal performance of the heat exchanger couple. The results show that there is no one-to-one correspondence of the minimum entropy generation rate and the maximum heat transfer rate. This indicates that the minimum entropy generation principle cannot be used for optimizing the heat exchanger couple.

Entransy—A physical quantity describing heat transfer ability

A new physical quantity, E h = 1 2 Q vh T , has been identified as a basis for optimizing heat transfer processes in terms of the analogy between heat and electrical conduction. This quantity, which will be referred to as entransy, corresponds to the electric energy stored in a capacitor. Heat transfer analyses show that the entransy of an object describes its heat transfer ability, as the electrical energy in a capacitor describes its charge transfer ability. Entransy dissipation occurs during heat transfer processes as a measure of the heat transfer irreversibility. The concepts of entransy and entransy dissipation were used to develop the extremum principle of entransy dissipation for heat transfer optimization. For a fixed boundary heat flux, the conduction process is optimized when the entransy dissipation is minimized, while for a fixed boundary temperature the conduction is optimized when the entransy dissipation is maximized. An equivalent thermal resistance for multi-dimensional conduction problems is defined based on the entransy dissipation, so that the extremum principle of entransy dissipation can be related to the minimum thermal resistance principle to optimize conduction. For examples, the optimum thermal conductivity distribution was obtained based on the extremum principle of entransy dissipation for the volume-to-point conduction problem. The domain temperature is substantially reduced relative to the uniform conductivity case. Finally, a brief introduction on the application of the extremum principle of entransy dissipation to heat convection is also provided.