Begell House Inc.
Computational Thermal Sciences: An International Journal
CTS
1940-2503
2
5
2010
HEAT EXCHANGERS USED IN REFRIGERATION CIRCUITS−MODELING AND EXPERIMENTAL VALIDATION
397-412
Fatma Salman
Marhoon
University of Bahrain
Peter R.
Senior
School of Chemical Engineering and Analytical Science, The University of Manchester, PO Box 88, Sackville Street, Manchester, M60 1QD, UK
Peter J.
Heggs
School of Chemical and Process Engineering, University of Leeds, Leeds, UK; Dept of Chem Eng, UMIST, Sackville St., Manchester, M60 1QD
The increased emphasis on energy savings and environmental protection has resulted in much more attention being paid toward the modeling of heating, ventilation, and air-conditioning (HVAC) equipment. These models permit the prediction of the system performance and the optimization of the system components during design. In addition, they can be used to develop new air-conditioning techniques and also allow the modification of the energy efficiency of an existing vapor compression system. Heat exchangers are widely used in the process industries. In the HVAC sector, they are integral to the performance of a vapor compression system. In addition, these have the most potential for modification in the design of HVAC systems. The main objective of this report is to develop performance models for two-phase heat exchangers, i.e., evaporators and condensers that are used in refrigeration circuits, and use them as tools for improving the energy usage. Steady state models have been developed for the heat exchangers in a 5 kW cold storage refrigeration unit, i.e., a flooded evaporator (thermosyphon heat exchanger) and an air-cooled condenser. Experimental data have been collected at various cold storage temperatures, namely, 5, 0, −10, and −20° C. The model for the flooded evaporator provides predictions of the following outlet conditions: the spatial averages of the temperatures of the air and refrigerant, and the outlet pressure and vapor quality of the refrigerant. In addition, the overall heat duty, the length of each region and the temperature distribution in each flow path, and the three-dimensional temperature distributions on the air side are detailed. The simulator predictions are in reasonable agreement with both the design and the experimental data to within an error of 20%. These models can be used in the redesign of heat exchangers in refrigeration systems for the newly mandated environmentally friendly refrigerants, and to meet the increasing regulatory minimum system efficiencies.
VALIDATION OF NUMERICAL SOLUTION OF THE STEFAN PROBLEM BY THE EXAMPLE OF MELT CRYSTALLIZATION
413-419
Vladimir
Ginkin
State Scientific Center of Russian Federation, Institute for Physics and Power Engineering (IPPE), Russia
Olga
Ginkina
State Scientific Center of Russian Federation, Institute for Physics and Power Engineering (IPPE), Obninsk, 249033, Russia
Svetlana
Ganina
State Scientific Center of Russian Federation, Institute for Physics and Power Engineering (IPPE), Obninsk, 249033, Russia
Kirill
Chernov
State Scientific Center of Russian Federation, Institute for Physics and Power Engineering (IPPE), Obninsk, 249033
The mathematical model of the Stefan problem solution in enthalpy variables together with convective heat and mass transfer in the liquid phase is described. The calculation results of experimental melt crystallization benchmarks are given.
THERMAL RECEPTIVITY OF FREE CONVECTIVE FLOW FROM A HEATED VERTICAL SURFACE: NONLINEAR WAVES
421-437
Michael
Wilson
Department of Mechanical Engineering University of Bath Bath BA2 7AY United Kingdom
D. Andrew S.
Rees
Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
Manosh C.
Paul
Systems, Power & Energy Research Division, School of Engineering, University of Glasgow, Glasgow, G12 8QQ, UK
In a previous paper (Int. J. Thermal. Sci., vol. 47, pp. 1382-1392, 2008), the authors performed a detailed numerical investigation of the linear instability of the thermal boundary layer flow over a vertical surface by introducing unsteady thermal disturbances near the leading edge and by solving numerically the fully elliptic linearized stability equations. The main aim of the present paper is to extend those results into the nonlinear regime by seeding the boundary layer with similar disturbances of finite amplitude. The ensuing nonlinear waves are found to exhibit a variety of behaviours, depending on the precise amplitude and period of the forcing. When the amplitude is sufficiently small, the linearized theory of the previous work is reproduced, but for larger amplitudes, cell splitting or cell merging may occur as waves travel downstream. Cell splitting takes place when disturbance frequencies are somewhat smaller than the most strongly amplified nondimensional disturbance frequency of 0.4 for which the boundary layer response, is at its greatest in terms of the surface rate of heat transfer (see Fig. 8 in previous paper). Cell merging takes place at frequencies what are approximately double that of the most strongly amplified disturbance frequency. Attention is focussed on fluids with a unit Prandtl number.
SPLITTING THE CONTRIBUTIONS OF VELOCITY AND VELOCITY GRADIENT TO THE TRANSPORT OF HEAT FLUX IN LAMINAR CONVECTION THROUGH A SQUARE DUCT WITH UNIFORM WALL TEMPERATURE
439-454
Liang-Bi
Wang
School of Mechanical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China; Key Laboratory of Railway Vehicle Thermal Engineering of MOE, Lanzhou Jiaotong University, Lanzhou,
Gansu 730070, PR China
Zhi-Min
Lin
School of Mechanical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China; Key Laboratory of Railway Vehicle Thermal Engineering of MOE, Lanzhou Jiaotong University, Lanzhou,
Gansu 730070, PR China
KeWei
Song
Department of Mechanical Engineering, Lanzhou Jiaotong University, 88 West Anning Rd. Anning District, Lanzhou 730070, Gansu, China
Xiang
Wu
Department of Mechanical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070 PRC
Kun
Hong
Department of Mechanical Engineering, Lanzhou Jiaotong University
The convective heat transfer process can be described through a process parameter, heat flux defined by Fourier's law, which results in the convective transport equation of heat flux. To find more support for the efficiency of using a heat flux transport equation, this paper (i) reports the heat flux transport equation in more a general convective heat transfer case, (ii) discusses conservation of heat flux on a control volume, and (iii) splits contributions of velocity and velocity gradient to the transport of heat flux in laminar convection through a square duct with uniform wall temperature. The main results can be summarized as follows. In a more general case, the transport equation of heat flux has source terms related to the gradient of thermal conductivity, the difference of thermal conductivity with respect to time, and the gradient of (ρcp) or (ρc). Furthermore, the volumetric integrations of the diffusion term and the convection term caused by velocity are related to integrations of fluxes through the boundaries of a control volume; other terms are the volumetric sources of the conserved quantity. Third, for laminar convection through a square duct, the contributions of velocity and velocity gradient to the convective transport of heat flux can be split.
HEAT TRANSFER IN THIN LIQUID FILMS FLOWING DOWN HEATED INCLINED GROOVED PLATES
455-468
Hongyi
Yu
Darmstadt University of Technology
Karsten
Loffler
Chair of Technical Thermodynamics, Darmstadt University of Technology, Petersenstr. 30, 64287 Darmstadt, Germany
Tatiana
Gambaryan-Roisman
Institute of Technical Thermodynamics and Center of Smart Interfaces, Technische Universitat Darmstadt, Alarich-Weiss-Str. 10, 64287, Darmstadt, Germany
Peter
Stephan
Institute for Technical Thermodynamics, Technische Universität Darmstadt, 64287 Darmstadt, Germany
Thin liquid films flowing down vertical or inclined plates are widely used in many industrial apparatuses. Using structured plate surfaces often leads to heat transfer enhancement. In the present work, a numerical model for heat transfer in a thin liquid film flowing down a heated, inclined, and grooved plate is developed. To this end, the Graetz-Nusselt problem for falling films on structured plates is solved. The computed velocity field and the developed temperature field, as well as the temperature distribution in the thermal entrance region of a falling film, are presented. The dependence of the temperature distribution on the Reynolds number, Biot number, applied heat flux, plate inclination angle, and plate topography is investigated. It is shown that the film rupture on the groove crests, which is observed in experiments at relatively high heat fluxes, can be attributed to the strong interface temperature gradients developing in the thermal entrance region. To qualitatively validate the numerical model, the hydrodynamics and heat transfer in falling films on structured plates are studied experimentally and simulated using the CFD tool FLUENT. The numerical results of the Graetz-Nusselt problem are discussed and compared with the experimental values and results of the FLUENT simulations.
UNSTEADY NATURAL CONVECTION IN AN ANISOTROPIC POROUS MEDIUM BOUNDED BY FINITE THICKNESS WALLS
469-485
Hosni Souheil
Harzallah
The National School of Engineers - University of Monastir, 5019, Tunisia
Abdelaziz
ZEGNANI
University of Gafsa, National Engineering School of Gafsa
Hacen
Dhahri
Laboratory of Thermal and Energy Systems Studies, National School of Engineers, Monastir
University, Monastir, Tunisia
Khalifa
Slimi
ISTLS
Abdallah
Mhimid
University of Monastir, Thermal System Energetic Laboratory Research (LESTE), National
School Engineering of Monastir, Avenue Ibn Jazzar, 5019 Tunisia
In this paper, a numerical study of unsteady natural convection in a fluid-saturated porous medium bounded by two equal-thickness walls has been made. The porous medium is assumed to be both hydrodynamically and thermally anisotropic. The vertical walls are isothermal at different temperatures, the horizontal walls are adiabatic. For modeling fluid flow inside the porous material, the Darcy flow model is assumed to hold. For heat transfer, we assume the validity of the local thermal equilibrium assumption. The classical finite volume method is used to solve the resulting dimensionless governing equations. Satisfactory agreement was obtained between results that validate the used computer code. The governing parameters considered for the analysis are the permeability ratios, Rpx and Rpz, the thermal conductivity ratios, Rcx and Rcz, the wall-to-porous medium heat capacity, σω, the wall-to-porous thermal conductivity ratio, Rω, and the ratio of the wall thickness to its height, D. The results show that lower values of Rcz, and/or higher values of Rpx, have negligible effects on the heat transfer rate but they are strongly subordinate to Rcx, Rpz, Rω, σω and D. Moreover, small values of Rcx, Rω, and σω, or larger values of Rpz and Rω, enhance convection inside the enclosure.