The property diagram in heat transfer and its applications

Abstract

Inspired by the property diagrams in thermodynamics, which distinctly reflect the performance and characteristics of thermodynamic cycles, we establish a state equation for heat motion and introduce a two-dimension property diagram, T-q diagram, in heat transfer to analyze and optimize the performance of heat exchangers, where heat flow is a state parameter for heat motion. According to the property diagram, it is convenient to obtain the influences of heat exchanger area, heat capacity rate and flow arrangement on the heat transfer performance during the analysis of heat exchangers and their networks. For instance, when analyzing the heat exchanger network in a district heating system, it is obvious to find that: if both the heat demand and the indoor air temperature in each branch of the network are the same, the total area of heat exchangers, the flow rate of water and the return water temperature in each branch are all the same; if the indoor air temperatures in different branches are different, the temperatures of the waters after flowing through different branches are different, which means that the mixing process of return waters with the same temperature is not an essential requirement to realize the best performance of district heating systems. Heat is one of the most common forms during energy utilization [1][2][3][4]. It can be obtained from the chemical energy of fuels through combustion, the atomic energy of radioactive material by either nuclear fission or nuclear fusion, and the solar energy or the geothermal energy by direct collection. Meanwhile, there are two different purposes in heat utilization: one is to generate power, and the other is to directly heat or cool objects. In the former purpose, heat is converted to mechanical energy through the heat absorption, expansion, heat release and compression processes of working substances, where the property diagrams in thermodynamics, e.g. p-v, T-s and h-s diagrams, are wildly used to directly express the variation rules of the state parameters of the working substances during a heat-work conversion process, qualitatively describe the exchanges of work and heat between the working substances and the surrounding environment in a thermodynamic cycle, and optimal design some ideal thermodynamic cycles for promoting the conversion efficiency. A thermodynamically reversible cycle is composed of several quasi-equilibrium processes, i.e. quasi-static processes, which have to progress at an infinitely slow rate in order to be reversible. Therefore, the output power of an ideal reversible thermodynamic cycle is zero. That is, a reversible cycle is a theoretical construct, which cannot actually occur and is seldom even approached in reality. In practical thermodynamic cycles, all real processes take place with finite potential difference and have finite rates, so they are all irreversible, where the property diagram in thermodynamics cannot be directly used to analyze and optimize the performance. For instance, heat transfer processes with finite temperature...

Analysis of heat transfer and irreversibility of organic rankine cycle evaporator for selecting working fluid and operating conditions

“…Chen et al [16] expressed that in the T-Q property diagram, which shows the heat transfer performance of a two-flow heat exchanger, the area between the hot fluid line and cold fluid line stands for the entransy dissipation due to the heat flow transferred from the hot fluid to the cold fluid. When the position of pinch point is at the start point of preheating (C r ≥ 1), the vaporization temperature of working fluid T w,evap , and the temperature of heat source fluid T h,evap at the starting point of vaporization of working fluid can be calculated from:…”

Thermo-economic investigation: an insight tool to analyze NGCC with calcium-looping process and with chemical-looping combustion for CO2capture

“…In this paper, a T-Q diagram, which is the change of heat as the abscissa and the temperature as the ordinate, is investigated owing to the following reasons: (i) it can easily catch the minimum and maximum temperature differences in a heat exchanger network; (ii) the area between hot composite curve and cold composite curve represents the entransy dissipation, which is used to analyze the heat transfer performance [39]; and (iii) it can graphically display the changes of heat and temperature. Although the components, energy, and mass are in balance, the HRSG system should be examined to avoid the minimum temperature difference below pinch point.…”

“…For the NGCC-PRE process, the energy donor is referring to the high temperature flue gas; for the NGCC-CLP and NGCC-CLC processes, both of the energy donors are additionally including CO 2 stream. In this paper, a T-Q diagram, which is the change of heat as the abscissa and the temperature as the ordinate, is investigated owing to the following reasons: (i) it can easily catch the minimum and maximum temperature differences in a heat exchanger network; (ii) the area between hot composite curve and cold composite curve represents the entransy dissipation, which is used to analyze the heat transfer performance [39]; and (iii) it can graphically display the changes of heat and temperature.…”

An optimization method for gas refrigeration cycle based on the combination of both thermodynamics and entransy theory

“…Based on the definitions of entransy and entransy dissipation [30,33,34], the variation of entransy flux of specific flow in the heat exchanger equals to the entransy flux at the inlet of the flow minus that at the outlet:…”

“…Furthermore, they proposed an entransy dissipation-based method for optimization of heat transfer elements [25][26][27][28][29], heat exchangers [30][31][32][33][34], and heat exchanger networks [35][36][37] in practical engineering applications. Besides, Chen et al extended entransy theory to analyze and optimize mass transfer [38] and coupled heat and mass transfer processes [39][40][41] based on the analogy between heat and mass transfer.…”