A new approach to analysis and optimization of evaporative cooling system I: Theory

Conservation equations of sensible entarnsy and latent entransy are established for flue gas convective heat transfer with condensation in a rectangular channel and the entransy dissipation expression is deduced. The field synergy equation is obtained on the basis of the extremum entransy dissipation principle for flue gas convective heat transfer with condensation. The optimal velocity field is numerically obtained by solving the field synergy equation. The results show that the optimal velocity field has multiple longitudinal vortices, which improve the synergy not only between the veloctiy and temperature fields but also between the velocity and vapor concentration fields. Therefore, the convective heat and mass transfers are significantly enhanced. Flow with multiple longitudinal vortices close to the optimal velocity field can be generated by discrete double-inclined ribs set in the rectangular channel. The numerical results show that the total heat transfer rate in the discrete double-inclined rib channel increases by 29.02% and the condensing heat transfer rate increases by 27.46% for Re = 600 compared with the plain channel. condensation, entransy, entransy dissipation, field synergy, longitudinal vortex Citation:Song W M, Meng J A, Li Z X. Optimization of flue gas convective heat transfer with condensation in a rectangular channel.

Section: Entransy and Entransy Dissipationmentioning

confidence: 99%

Optimization of flue gas convective heat transfer with condensation in a rectangular channel

Song

Jiang

2011

“…Chen et al [74][75][76][77] proposed the mass entransy dissipation (ever interpreted as mass transfer potential capacity dissipation function in [74]) extremum principle, and optimized various processes, such as laminar mass transfer [74], photocatalytic oxidation reactor, space station ventilation [76,77] and evaporative cooling systems [78][79][80], etc. Chen et al [81] proposed the moisture resistance method for liquid desiccant performance analysis and optimization.…”

Section: Mass Transfermentioning

confidence: 99%

Progress in entransy theory and its applications

Chen

2012

115

The entransy and entransy dissipation extremum principle proposed have opened up a new direction for the heat transfer optimization. The emergence and development of entransy theory are reviewed. Entransy theory and its applications are summarized from several aspects, such as heat conduction, heat convective, heat radiation, heat exchanger design and mass transfer, etc. The emphases are focused on four aspects, i.e., the comparison between entropy generation rate and entransy dissipation rate, the combination of entransy dissipation extreme principle with finite time thermodynamics, the combination of entransy dissipation extreme principle with the heat conduction constructal theory, and the combination of entransy dissipation extreme principle with the heat convective constructal theory. The scientific features of entransy theory are emphasized. Influenced by oil crisis in the 1970's directly, the heat transfer enhancement was world-widely paid attention by the scientific community, and was quickly developed into one of very important branches in thermal science and technology fields. With the progress of social development ideas, such as sustainable development, low-carbon technology and low-carbon economy, the concept of heat transfer enhancement has developed into a broader scope concept of heat transfer optimization [1]. In the past 30 years, the basic theory of heat transfer optimization has been made considerable progresses, and finite-time thermodynamics, field synergy principle, constructal theory as well as entransy theory are the representative achievements. The emergences of these theories promoted the developments of thermodynamics and heat transfer. Finite-time thermodynamics, which combined thermodynamics, heat transfer and fluid mechanics, provided a theoretical fundament for performance optimizations of practical processes and devices with the constraints of a finite size and finite time. Field synergy principle unified the essential physical understandings of convective heat transfer and heat transfer enhancement phenomena, and provided a theoretical fundament for the development of heat transfer enhancement technology to achieve better energy saving effects. Constructal theory, which was called the thermodynamics of non-equilibrium system with configuration in the field of thermal science, provided a theoretical fundament for the unified descriptions of the generation of flow structures in natural organization and the designs of various flow structures. Fourier's law of heat conduction, Newton's law of cooling and Stephen-Boltzmann law of blackbody radiation only gave the concept of heat transfer rate, while the concepts of entransy and entransy dissipation lay the foundation of defining the concept of heat transfer efficiency which did not appear in the heat transfer theory before. Entransy theory provided a new theoretical fundament for heat transfer optimization, which is different from that of the entropy generation minimization.

“…This has already been successfully applied to analyze and optimize heat and mass transfer processes and apparatus including heat conduction [5][6][7][8], heat convection [9][10][11][12][13][14], thermal radiation [15], evaporative cooling [16,17] and heat exchanger (networks) [18][19][20][21][22][23]. In this article, we briefly review the origin and applications of the property diagrams in thermodynamics, propose two state parameters in heat transfer based on the analogy between heat transfer and fluid flow, construct a property diagram in heat transfer by taking the two state parameters as coordinates to describe the variations of entransy for both hot and cold fluids during heat transfer processes and meanwhile measure the irreversibility of the processes, and finally to optimize heat transfer performance and promote energy utilization efficiency qualitatively.…”

mentioning

confidence: 99%

The property diagram in heat transfer and its applications

Chen

Zhang

2012

Self Cite

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...