共查询到20条相似文献,搜索用时 31 毫秒
1.
In the present communication, the unglazed cylindrical solar adsorber module is suggested for refrigeration and theoretical models for the heat and mass transfer in the cylindrical adsorber with heat balance equations in the collector components have been developed. It has been found that, both the SCP and COPsolar raises with increasing the evaporation temperature and drop off with the increase of the condensation temperature. The COPsolar increased from 0.15 to 0.52 with the increase of the total solar energy absorbed by the collector while the COPcycle varied in the range of 0.57–0.73. The efficiency of unglazed solar collector varied from 36 to 44 %. The cost of current unglazed adsorption refrigeration system is compared with the glazed system, and it is 33 to 50 % less than the cost of glazed system. 相似文献
2.
For the adsorbents in the application of refrigeration, the density of the material inside the adsorber changes because the adsorption/desorption of the refrigerant inside the adsorbents. Consequently the thermal conductivity and permeability of the adsorbents also change. In order to investigate the heat and mass transfer performance of consolidated compound adsorbent under the different equilibrium state of adsorption/desorption, the thermal conductivity and permeability test system is set up using the guarded hot plate measuring method and the principle of Ergun equation. Then various mass ratios between adsorbent and matrix of consolidated physical and chemical compound adsorbents are developed and tested under different ammonia adsorption quantity. Result shows that the thermal conductivity and permeability have strong dependence with the ratios and consolidated density of the compound adsorbent. Meanwhile, the thermal conductivity and permeability of the chemical compound adsorbents vary significantly with different adsorption quantity of ammonia, and the values for the physical compound adsorbents almost maintain a constant value with different values of adsorption quantity. 相似文献
3.
This paper presents isosteric-based adsorption equilibrium tests of three activated carbon samples with methanol as an adsorbate.
Experimental data was fitted into Langmuir equation, Freundlich equation and Dubinin-Astakov (D–A) equation, respectively.
The fitted adsorption equations were compared in terms of agreement with experimental data. Moreover, equation format’s impacts
on calculation of the coefficient of performance (COP) and refrigeration capacity of an adsorption refrigeration system was
analyzed. In addition, the sensitivity of each parameter in each adsorption equation format to the estimation of cycle’s COP
and refrigeration capacity was investigated. It was found that the D–A equation is the best form for presenting the adsorptive
property of a carbon-methanol working pair. The D–A equation is recommended for estimating thermal performance of an adsorption
refrigeration system because simulation results obtained using the D–A equation are less sensitive to errors of experimentally
determined D–A equation’s parameters. 相似文献
4.
M. A. Rady 《Heat and Mass Transfer》2009,45(5):545-561
Enhanced efficiency of the adsorption process in the dehumidifier is a key element for improved performance of desiccant cooling
systems. Due to the exothermic nature of the adsorption process, the dehumidification and cooling capacity are limited by
significant temperature changes in the adsorption column. In the present study, the effects of integration of sensible and
latent heat storage particles in the desiccant bed for in situ management of released adsorption heat are investigated. For
this purpose, column experiments are performed using an initially dry granular bed made of silica-gel particles or a homogeneous
mixture of silica gel and inert sensible or latent heat storage particles. The packed bed is subject to a sudden uniform air
flow at selected values of temperature and humidity. Also, a packed bed numerical model is developed that includes the coupled
non-equilibrium heat and moisture transfer in the solid and gas phases. Investigations of the heat and mass transfer characteristics
are reported using the composite structure and the results are compared with the base case of simple silica gel bed. Improved
desiccant cooling system performance can be obtained by appropriate adjustment of desiccant cycle operation and proper choice
of the volume ratio of thermal energy storage particles. 相似文献
5.
The working pair zeolite-water has very good characteristics for the heat pump application. It is non-poisonous, non-flammable
and low-corrosive so that the use of a zeolite-water heat pump in the large field of domestic heating is very promising.
The poor heat and mass transfer of the zeolite has to be considered by an appropriate design of the adsorber heat exchanger.
Compact zeolite layers directly linked with the heat exchanger enable a high specific thermal output (thermal output related
to the mass of zeolite) which is the main shortcoming of these machines. Additionally the coefficient of performance (COP)
can be improved significantly by a modular design of the machine consisting of six to eight heat pump modules. Due to the
periodical operating mode which is required by the zeolite-water pair the single module is built up in a simple way without
any moving parts. The different modules, each of them operating in another phase of the sorption cycle, are connected in series
by a heat transfer medium circuit so that a continuous thermal output together with high COP is achieved by this zeolite-water
heat pump.
First experimental investigations focus on the layout of the different components of the heat pump, e.g. the single module,
the adsorber/desorber and the evaporator/condenser. The paper will present the design of these components as well as the design
of the entire modular machine. Furthermore there will be a theoretical discussion of the COPs of the modular heat pump depending
on the ambient temperature, on the number of modules and on the heating system.
Received on 12 November 1998 相似文献
6.
The results of solving the one-dimensional problem of the motion of a pulse of radioactive gas, carried through a porous medium by a stream of inert carrier with constant velocity, are generalized by the case of taking diffusion processes into account. For a delta-shaped input pulse, the solution is obtained of a system of equations which describes the migration of the pulse, taking diffusion washout and nonequilibrium adsorption into account. It is shown that in the case of equilibrium adsorption the time of appearance of the concentration maximum at the adsorber outlet depends on the decay constant and the coefficient of diffusion. Approximate solutions for strong- and weak-nonequilibrium adsorption and in the case of weak diffusion are considered. An estimate is carried out of the maximum magnitude of the diffusion coefficient, when its effect can be neglected.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 85–90, September–October, 1976.The authors thank A. N. Gudkova for discussing the work. 相似文献
7.
A general method for the investigation of adsorption kinetics of nonlinear,nonisothermal systems, based on frequency response analysis, ispresented. It is based on the definition of higher-order frequency response functions (FRFs) on the adsorber and on the particle level. FRFs on the adsorber level can be estimated from experimentally measuredadsorber FR and used to calculate FRFs on the particle level,which can be further used for model identification, by comparison withtheoretical particle FRFs. The general procedure for calculation of particle FRFs from those of adsorber is given. Also, the generalprocedure for theoretical derivation of particle FRFs is given andillustrated with an example of nonisothermal adsorption governed bymicropore diffusion and film resistance heat transfer, as well as theprocedure for calculating unmeasured adsorber FRFs, which isillustrated with an example of a batch adsorber with volume modulation. 相似文献
8.
Narrow channel heat transfer technique is a new developing heat transfer technique in recent years. As the temperature of droplet, steam and wall are decided by forced convection heat transfer between the steam and the wall, between the droplet and the wall, between the steam and the droplet and radiation heat transfer, which makes heat transfer mechanism of dispersed flow be difficultly interpretative. Dispersed flow in narrow annular channel is analyzed in the paper, investigating the influence of all kinds of heat transfer processes on dispersed flow, building annular channel dispersed flow model using thermodynamic non-equilibrium model. Calculation results show heat transfer is mainly controlled by heat transfer process between steam and wall. When temperature is low, radiation can be ignored on heat transfer coefficient calculation. The calculation of model can provide a reference for engineering application of steam generator, refrigeration system and so on. 相似文献
9.
Absorber is an important component in vapor absorption refrigeration system and its performance has greater influence in overall
efficiency of absorption machines. Falling film heat and mass transfer in an absorber is greatly influenced by fluid properties,
geometry of heat exchanger and its operating parameters. This paper presents on the results of experimental studies on the
heat and mass transfer characteristics of a coiled tube falling film absorber, using 1,1,1,2-Tetrafluroethane(R-134a) and
N-N Dimethyl Acetamide (DMAC) as working fluids. The effects of film Reynolds number, inlet solution temperature and cooling
water temperature on absorber heat load, over all heat transfer coefficient and mass of refrigerant absorbed are presented
and discussed. Normalized solution and coolant temperature profiles and refrigerant mass absorbed along the height of absorber
are also observed from the experimental results. The optimum over all heat transfer coefficient for R-134a–DMAC solution found
to be 726 W/m2K for a film Reynolds number of 350. The R-134a vapour absorption rate is maximum in the normalized coil height of 0.6 to
1. 相似文献
10.
An approach to describe heat transfer in porous media is presented on the basis of the continuous time random walk (CTRW)
framework. CTRW is capable of quantifying both local equilibrium and non-equilibrium heat transfer in heterogeneous domains,
and is shown here to match published experimental data of non-equilibrium thermal breakthrough. It is argued that CTRW will
be particularly applicable to the quantification of heat transfer in naturally heterogeneous geological systems, such as soils
and geothermal reservoirs. 相似文献
11.
During the past two decades the thermoacoustic refrigeration and prime mover cycles gained importance in a variety of refrigeration
applications. Acoustic work, sound, can be used to generate temperature differences that allow the transport of heat from
a low temperature reservoir to an ambient at higher temperature, thus forming a thermoacoustic refrigeration system. The thermoacoustic
energy pumping cycle can also be reversed: temperature difference imposed along the stack plates can lead to sound generation.
In this situation the thermoacoustic system operates as a prime mover. Sound generated by means of this thermoacoustic energy
conversion process can be utilized to drive different types of refrigeration devices that require oscillatory flow for their
operation, such as thermoacoustic refrigerators, pulse tubes and Stirling engines. In order for a thermoacoustic refrigeration
or prime mover system as well as a thermoacoustic prime mover driving a non-thermoacoustic refrigeration system to be competitive
on the current market, it has to be optimized in order to improve its overall performance. Optimization can involve improving
the performance of the entire system as well as its components. The paper addresses some of the thermodynamic and heat transfer
issues relevant in improving the performance of the thermoacoustic system, such as optimization for maximum COP, maximum cooling
load and the role of the heat exchangers. Results obtained using the two optimization criteria are contrasted in the paper
to illustrate the complexity of the optimization process. 相似文献
12.
With a previously developed numerical model, we perform a detailed study of the heat extraction process in enhanced or engineered geothermal system (EGS). This model takes the EGS subsurface heat reservoir as an equivalent porous medium while it considers local thermal non-equilibrium between the rock matrix and the fluid flowing in the fractured rock mass. The application of local thermal non-equilibrium model highlights the temperature-difference heat exchange process occurring in EGS reservoirs, enabling a better understanding of the involved heat extraction process. The simulation results unravel the mechanism of preferential flow or short-circuit flow forming in homogeneously fractured reservoirs of different permeability values. EGS performance, e.g. production temperature and lifetime, is found to be tightly related to the flow pattern in the reservoir. Thermal compensation from rocks surrounding the reservoir contributes little heat to the heat transmission fluid if the operation time of an EGS is shorter than 15 years. We find as well the local thermal equilibrium model generally overestimates EGS performance and for an EGS with better heat exchange conditions in the heat reservoir, the heat extraction process acts more like the local thermal equilibrium process. 相似文献
13.
Peter C. Ma Tim Ewan Christopher Jainski Louise Lu Andreas Dreizler Volker Sick Matthias Ihme 《Flow, Turbulence and Combustion》2017,98(1):283-309
The performance, efficiency and emissions of internal combustion (IC) engines are affected by the thermo-viscous boundary layer region and heat transfer. Computational models for the prediction of engine performance typically rely on equilibrium wall-function models to overcome the need for resolving the viscous boundary layer structure. The wall shear stress and heat flux are obtained as boundary conditions for the outer flow calculation. However, these equilibrium wall-function models are typically derived by considering canonical flow configurations, introducing substantial modeling assumptions that are not necessarily justified for in-cylinder flows. The objective of this work is to assess the validity of several model approximations that are commonly introduced in the development of wall-function models for IC-engine applications. This examination is performed by considering crank-angle resolved high-resolution micro-particle image velocimetry (µ-PIV) measurements in a spark-ignition direct-injection single cylinder engine. Using these measurements, the performance of an algebraic equilibrium wall-function model commonly used in RANS and LES IC-engine simulations is evaluated. By identifying shortcomings of this model, a non-equilibrium differential wall model is developed and two different closures are considered for the determination of the turbulent viscosity. It is shown that both wall models provide adequate predictions if applied inside the viscous sublayer. However, the equilibrium wall-function model consistently underpredicts the shear stress if applied in the log-layer. In contrast, the non-equilibrium wall model provides improved predictions of the near-wall region and shear stress irrespective of the wall distance and the piston location. By utilizing the experimental data, significant adverse pressure gradients due to the large vortical motion inside the cylinder (induced by tumble, swirl and turbulence) are observed and included in the non-equilibrium wall model to further improve the model performance. These investigations are complemented by developing a consistent wall heat transfer model, and simulation results are compared against the equilibrium wall-function model and Woschni’s empirical correlation. 相似文献
14.
The heat and mass transfer in an unsaturated wet cylindrical porous bed packed with quartz particles was investigated theoretically for relatively low convective drying rates. Local thermodynamic equilibrium was assumed in the mathematical model describing the multi-phase flow in the unsaturated porous media using the energy and mass conservation equations to describe the heat and mass transfer during the drying. The drying model included convection and capillary transport of the free water, diffusion of bound water, and convection and diffusion of the gas. The numerical results indicated that the drying process could be divided into three periods, the temperature rise period, the constant drying rate period and the decreasing drying rate period. The numerical results agreed well with the experimental data verifying that the mathematical model can evaluate the drying performance of porous media for low drying rates. The effects of drying conditions such as the ambient temperature, the relative humidity, and the velocity of the drying air, on the drying process were evaluated by numerical solution. 相似文献
15.
Z. B. Yan F. Duan T. N. Wong K. C. Toh K. F. Choo P. K. Chan Y. S. Chua L. W. Lee 《Heat and Mass Transfer》2013,49(7):985-990
An inclined spray chamber with four multiple nozzles to cool a 1 kW 6U electronic test card has been designed and tested in this study. The multiple inclined sprays can cover the same heated surface area as that with the multiple normal sprays but halve the volume of the spray chamber. The spray cooling system used R134a as a working fluid in a modified refrigeration cycle. It is observed that increasing mass flow rate and pressure drop across the nozzles improved the heat transfer coefficient with a maximum enhancement of 117 %, and reduced the maximum temperature difference at the heated surface from 13.8 to 8.4 °C in the inclined spray chamber with a heat flux of 5.25 W/cm2, while the heat transfer coefficient of the normal spray increased with a maximum enhancement of 215 % and the maximum temperature difference decreased from 10.8 to 5.4 °C under similar operating conditions. We conclude that the multiple inclined sprays could produce a higher heat transfer coefficient but with an increase in non-uniformity of the surface temperature compared with the multiple normal sprays. 相似文献
16.
Kyudae Hwang Chan Ho Song Sung Ki Kim Kiyoshi Saito Sunao Kawai 《Heat and Mass Transfer》2013,49(3):359-367
Due to the concern on global warming, the demand for a system using natural refrigerant is increasing and many researches have been devoted to develop systems with natural refrigerants. Among natural refrigerant systems, an air cycle system has emerged as one of alternatives of Freon gas system due to environmentally friendly feature in spite of the inherent low efficiency. To overcome the technical barrier, this study proposed combination of multiple systems as a hybrid cycle to achieve higher efficiency of an air cycle system. The hybrid air cycle adopts a humidity control units such as an adsorber and a desorber to obtain the cooling effect from latent heat as well as sensible heat. To investigate the efficacy of the hybrid air cycle, the cooling performance of a hybrid air cycle is investigated analytically and experimentally. From the simulation result, it is found that COP of the hybrid air cycle is two times higher than that of the conventional air cycle. The experiments are conducted on the performance of the desiccant system according to the rotation speed in the system and displayed the feasibility of the key element in the hybrid air cycle system. From the results, it is shown that the system efficiency can be enhanced by utilization of the exhausted heat through the ambient heat exchanger with advantage of controlling the humidity by the desiccant rotor. 相似文献
17.
The present study is concerned with the phase change during rapid depressurization of fluids: the role of vapor bubbles nucleation and growth and the effect on the system fluid dynamics were modeled and experimental measurements were made. Following a control-volume approach, averaged equations governing the motion of a one-dimensional, homogeneous, no-slip two-phase flow were used considering both thermal equilibrium (equal temperature) and non-equilibrium (non-equal temperature) between the liquid and vapor phases. In the non-equilibrium model, the heat transfer from the liquid to the vapor and the corresponding mass transfer velocity were modeled. Model results were compared with experimental data for a loss-of-coolant accident in nuclear power plants: the comparison of numerical vs. experimental data showed the role of nucleation velocity during the earliest phase of rapid depressurization. The experimental study of two-phase flow in a diesel engine injection system has been carried out using a rotative pump which is operated by using a purpose-developed test-bench; pressure measurements inside the system pipes were performed using pressure transducers; moreover, an ultrasonic technique was employed to study phase change phenomena. Several measurements were performed comparing the results obtained by different experimental techniques with the model outputs.Sommario.presente studio riguarda il fenomeno della cavitazione durante la depressurizzazione di fluidi. E'stata considerata la velocità di formazione e nucleazione delle bolle di vapore e le equazioni di conservazione sono state integrate con solutori al 1°e 2°ordine di tipo ENO. Sono stati utilizzati dati sperimentali ottenuti durante incidenti per perdita di refrigerante in centrali nucleari; per quanto riguarda gli apparati di iniezione, gli autori hanno sviluppato due differenti tecniche sperimentali, basate rispettivamente sulla pressione e sulla riflessione degli ultrasuoni. Il confronto dei risultati numerici con quelli sperimentali è stato soddisfacente. 相似文献
18.
Double diffusive convection in a fluid-saturated rotating porous layer heated from below and cooled from above is studied when the fluid and solid phases are not in local thermal equilibrium, using both linear and non-linear stability analyses. The Darcy model that includes the time derivative and Coriolis terms is employed as momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for energy equation. The onset criterion for stationary, oscillatory and finite amplitude convection is derived analytically. It is found that small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of thermal and solute diffusions that causes the convection to set in through either oscillatory or finite amplitude mode rather than stationary. The effect of solute Rayleigh number, porosity modified conductivity ratio, Lewis number, diffusivity ratio, Vadasz number and Taylor number on the stability of the system is investigated. The non-linear theory based on the truncated representation of Fourier series method predicts the occurrence of subcritical instability in the form of finite amplitude motions. The effect of thermal non-equilibrium on heat and mass transfer is also brought out. 相似文献
19.
《Particuology》2022
The gas–liquid–solid mini fluidized bed (GLSMFB) combines the advantages of fluidized bed and micro-reactor, and meets the requirements for safety and efficiency of green development of process industry. However, there are few studies on its flow performance and no studies on its mass and heat transfer performance. In this paper, the characteristics of gas–liquid mass transfer in a GLSMFB were studied in order to provide basic guidance for the study of GLSMFB reaction performance and application. Using CO2 absorption by NaOH as the model process, the gas–liquid mass transfer performance of GLSMFB was investigated. The results show that the liquid volumetric mass transfer coefficient and the gas–liquid interfacial area both increase with the increase of the superficial gas velocity within the experimental parameter range under the same given superficial liquid velocity. At the same ratio of superficial gas to liquid velocity, the liquid volumetric mass transfer coefficient increases with the increase of the superficial liquid velocity. Fluidized solid particles strengthen the liquid mass transfer process, and the liquid volumetric mass transfer coefficient is about 13% higher than that of gas–liquid mini bubble column. 相似文献
20.
激波与边界层之间相互作用是高超声速飞行中的常见现象,对飞行器气动性能与飞行安全至关重要.对于高焓来流,流场中通常存在复杂的物理化学现象,此时准确模拟流场中激波边界层相互作用的难度大,相关物理化学建模仍有待进一步考察和研究.本文针对最近文献中纯净空气高超声速双锥绕流实验开展数值研究,分别研究了不同热化学模型与输运模型对壁面压力与热流的影响.热力学模型包括完全气体、热力学平衡和非平衡模型,化学模型包括冻结和非平衡化学模型,输运模型包括经典的Wilke/Blottner/Eucken模型与更加复杂的Gupta/SCEBD模型,以及考虑壁面催化/非催化影响的模型.计算了6个不同算例,涵盖了低焓至高焓来流等不同工况.壁面压力与热流的数值计算结果与实验结果符合较好;对于低焓来流,计算结果主要受到分子内能分布的影响,输运模型对计算结果的影响不大;对于高焓来流,一方面计算结果受到化学反应与壁面催化的影响较大,另一方面不同输运模型对计算结果的影响也更加明显. 相似文献