Moisture transfer resistance method for liquid desiccant dehumidification analysis and optimization

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.

Section: Mass Transfermentioning

confidence: 99%

Progress in entransy theory and its applications

Chen

2012

115

“…This has already been successfully applied to heat transfer processes and apparatuses [16][17][18][19] including heat conduction, convective heat transfer, thermal radiation and heat exchangers. Chen et al subsequently extended the entransy theory to analyze and optimize the mass transfer process [20] and such coupled heat and mass transfer processes as liquid desiccant dehumidification [21] and evaporative cooling [22]. Jiang et al [23] and Xie et al [24] proposed a series of theories and methods for optimizing heat and moisture transfer based on entransy theory, which already has some applications in evaporative cooling systems.…”

mentioning

confidence: 99%

Optimization criteria for the performance of heat and mass transfer in indirect evaporative cooling systems

Fang

Chen

2012

Self Cite

The wide application of evaporative cooling techniques in which the optimization criteria form the theoretical basis for improving evaporative cooling performance is essential for energy conservation and emission reduction. Based on exergy analysis and the entransy dissipation-based thermal resistance method, this contribution aims to investigate the effects of flow and area distributions in the optimization of the performance of indirect evaporative cooling systems. We first establish the relationships of exergy efficiency, entransy dissipation-based thermal resistance and cooling capacity of a typical indirect cooling system. Using the prescribed inlet parameters, the heat and mass transfer coefficients and the circulating water mass flow rate, we then numerically validate that when the cooling capacity reaches a maximum, the entransy dissipation-based thermal resistance falls to a minimum while the exergy efficiency is not at an extreme value. The result shows that the entransy dissipation-based thermal resistance, not the exergy efficiency, characterizes the heat transfer performance of an evaporative cooling system, which provides a more suitable method for evaluating and analyzing the indirect cooling system. indirect evaporative cooling, optimization criteria, entransy dissipation-based thermal resistance, exergy efficiency Citation: Yuan F, Chen Q. Optimization criteria for the performance of heat and mass transfer in indirect evaporative cooling systems. Chin Sci Bull, 2012, 57: 687693,Evaporative cooling is a technique in which unsaturated moist air is in contact with water, causing liquid water evaporation resulting in low temperature water or wind. Since the ambient air is clean and low cost, evaporative cooling is the focus of wide interest and applications. Many scientists have undertaken a large amount of research on evaporative cooling systems, mainly on the following three aspects: (1) The analysis of the impact factors on the performance of evaporative cooling systems, such as temperature, humidity and the mass flow rate of moist air [1,2], temperature and the mass flow rate of water [3] and direct/ indirect evaporative cooling with different structures, such as plate types [4,5], tubular types [6,7] and heat pipes [8] etc.(2) The design and optimization of evaporative cooling cycles, including two-stage or three-stage combined direct/ indirect evaporative cooling [9,10] and combined liquid desiccant dehumidification and evaporative cooling [11]. (3) The performance analysis and optimization theory of evaporative cooling systems, with emphasis on exergy analysis [12,13]. In exergy theory, researchers always take exergy efficiency as an assessment index to analyze the exergy loss distribution of all irreversible processes in the system. This involves evaluating the performance of evaporative cooling, and seeking effective ways for improving the performance of evaporative cooling using the criterion that maximum exergy efficiency leads to optimal performance. However, the physical essence of exer...

“…The heat transfer was significantly enhanced by longitudinal vortex flow. Chen et al [20] defined moisture transfer resistance and optimized liquid desiccant dehumidification. Recently, Liu et al [21] studied the physical quantity synergy in a laminar flow field and its application in heat transfer enhancement.…”

mentioning

confidence: 99%

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

Song

Jiang

2011

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.