Effect of electrohydrodynamic (EHD) on condensation of R-134a in presence of non-condensable gas

Effects of applying EHD and non-condensable (NC) gas contents have been experimentally studied on inter-tubular condensation of refrigerant R-134a flow. Applying of electrical field enhances condensing heat transfer coefficient (CHTC), but presence of NC gas in condensing vapour reduces this coefficient. In competition of these two effective parameters on condensation, it can be observed that at higher concentration of NC gas, the effect of electrical field on enhancement of CHTC is greatly reduced. But at lower concentration of NC gas, the effect of electrical field is more considerable, due to thickness of heat transfer boundary layer.     

A transient analysis of three-dimensional heat and mass transfer in a molten carbonate fuel cell at start-up

In this paper, we investigate the time-dependent heat and mass transfer in a molten carbonate fuel cell at start-up. Thus, a three-dimensional, transient mathematical model is presented through a comprehensive inclusion of various physical, chemical and electrochemical processes that occur within the different components of molten carbonate fuel cells. The model is proposed as a predictive tool to provide a three-dimensional demonstration of variable variations at system start-up. The local distribution of field variables and quantities are showcased. It reveals that the electrochemical reaction rate is dominated by the over-potential, not by the reactants' molar fraction. Reversible heat generation and consumption mechanisms of the cathode and anode are dominant in the first 10 s while the heat conduction from the solid materials to the gas phase is negligible. Meanwhile, activation and ohmic heating have nearly the same impact within the anode and cathode. Based on these findings, the importance of heat conduction and its main features are finally assessed.     

Heat transfer characteristics of microencapsulated phase change material slurry in laminar flow under constant heat flux

Due to its large apparent specific heat during the phase change period, microencapsulated phase change material slurry (MPCMS) has been suggested as a medium for heat transfer. In this paper, the convective heat transfer characteristics of MPCMS flowing in a circular tube were experimentally and numerically investigated. The enhanced convective heat transfer mechanism of MPCMS, especially in the thermal fully developed range, was analyzed by using the enthalpy model. Three kinds of fluid–pure water, micro-particle slurry and MPCMS were numerically investigated. The results show that in the phase change heat transfer region the Ste number and the Mr number are the most important parameters influencing the Nusselt number fluctuation profile and the dimensionless wall temperature. Re_b, d_p and c also influence the Nusselt number profile and the dimensionless wall temperature, but they are independent of phase change process.     

Numerical Modeling of the Solidification Phase Change in a Pipe and Evaluation of the Effect of Boundary Conditions

A numerical solution to a two-dimensional model of flow and transient heat transfer involving solidi- fication in a pipe has been established. Where the temperature of pipe wall is below the freezing point of fluid, phase change of flowing fluid and the influence of different boundary condition, such as pipe wall temperature, initial temperature and inlet velocity has been taken into account. Also it has been investigated to elicit proper non-dimensional numbers to show the solidification proceeding results. Additionally comparing the two acceptable inlet conditions, show distinctions between velocity inlet and pressure inlet boundary condition in such problems, which affect the whole freezing process.     

Numerical study of convective flow with condensation of a pure fluid in capillary regime

A stationary 2-phase flow model with condensation in the capillary regime, based on a separate flow approach was developed. One of the specificities of the model is that it takes into account the coupling between a cylindrical interface (region with a thin film of liquid) and a hemispherical interface (main meniscus at the end of the condensation region). A specific algorithm was developed for numerical resolution to overcome the difficulty related to the presence of a free boundary condition. Analysis of the liquid–vapour interface profiles and the various local parameters allowed us to establish the heat and mass transfer laws for the particular type of regime studied. We analysed the dominant effects of this type of flow, which are characterised by dimensionless numbers Ca (capillary number) and Bo (boiling number), representing the competition between the capillary, viscous and phase-change effects. The effects due to the difference in density between the two phases and to the Reynolds number were also studied. We show that the mean heat transfer coefficients are driven by the profile of the interface. Hence, in certain situations, even when the liquid film becomes thinner on average an unexpected lowering of the efficiency of heat transfer is obtained. These effects are closely related to the coupling between the thin liquid film region and the main meniscus.     

Validation of the use of heat transfer models in liquid/solid fluidized beds for ice slurry generation

Heat transfer coefficients in a liquid/solid fluidized bed heat exchanger are investigated for application in ice slurry generators. A range of temperature driving forces is determined in which ice slurry generation is stable. In this range ice crystal formation or growth does not affect heat transfer coefficients. A model is proposed that accurately predicts heat transfer coefficients in the fluidized bed ice slurry generator. Due to lower temperatures and higher viscosity in ice slurry generation, heat transfer coefficients measured are lower than predicted with heat transfer correlations specific for liquid/solid fluidized bed heat exchangers. Heat transfer coefficients measured are however significantly higher than for single phase fluid flow.     

Erratum to “Validation of the use of heat transfer models in liquid/solid fluidized beds for ice slurry generation” [International Journal of Heat and Mass Transfer 46 (2003) 3683-3695]

In the concerned paper, tube dimensions have been taken from the experimental set-up drawings. Recently, during Wilson plot calibration tests, after extension of the operating range of the set-up, came to light that the tube sizes used were slightly different from what was stated in the drawings. Consequently, published experimental fluidized bed heat transfer coefficients are up to 40% too low and some conclusions had to be adapted. The new, correct experimental heat transfer results show a much better agreement with heat transfer models in literature. The authors of the subjected paper deeply regret the errors. Corrected versions of Sections 4 and 5 of the paper are presented here.     

Heat transfer in fluctuating flow past an infinite plate with suction and constant heat flux

An analysis of unsteady heat transfer in a two-dimensional flow past an infinite porous plate has been carried out under the following conditions: (1) constant or variable suction; (2) free-stream oscillating in time about a non-zero constant mean; and (3) constant heat flux at the plate. Approximate solutions to the temperature field have been derived. the transient temperature, the amplitude and phase of the Nusselt number are shown on graphs.     

Modelling and numerical simulation of convection driven high pressure induced phase changes

High pressure induced freezing and thawing processes are investigated by means of modelling and numerical simulation. The enthalpy-porosity model is reformulated in terms of conservation equations of mass, momentum and heat for a compressible medium. It can be shown that convective fluid motion has a major influence on the formation of the ice front. Furthermore, due to a vanishing density anomaly under high pressure, natural convection exhibits a different behaviour than under low pressure thus the formation of the ice front under high pressure obeys different mechanisms than that under ambient pressure.     

Thermal efficiency analysis of the cascaded latent heat/cold storage with multi-stage heat engine model

The cascaded thermal storage technique has emerged as an important solution for efficient conversion and utilization of thermal energies. In this paper, an exergy optimization was performed for cascaded latent cold/heat storage using multi-stage heat engine model. The optimization solution for both heat storage and cold storage systems was obtained, which was used for guiding the selection of PCMs with two examples presented. Cascaded thermal storage with increased stage number can not only extend temperature band for multi-grade thermal energy, but also reduce the exergy of the outlet HTF. It was found that heat transfer enhancement (improving NTU) is very necessary for a cascaded thermal storage system. The COP of cold energy may be greater than 1, which is also higher than that of heat for the same temperature difference in a cascaded thermal storage system. The increased environment temperature improves the COP of the cascaded cold storage (from 0.54 to 0.68) but reduces that of the cascaded heat storage (from 0.42 to 0.366). In the practical design of the cascaded thermal storage system, the stage number should be determined by balancing economics and system complexity.     

Thermal management of metal hydride hydrogen storage reservoir using phase change materials

Metal hydride hydrogen storage reservoir should be carefully designed to achieve acceptable performance due to significant thermal effect on the system during hydriding/dehydriding. Phase change materials can be applied to metal hydride hydrogen storage system in order to improve the system performance. A transient two-dimensional axisymmetric numerical model for the metal hydride reservoir packed with LaNi₅ has been developed on Comsol platform, which was validated by comparing the simulation results with the experiment data from other work. Then, the performances of metal hydride hydrogen storage reservoir using phase change materials were predicted. The effects of some parameters, such as the thermal conductivity, the mass and the latent heat of fusion of the phase change materials, on the metal hydride hydrogen storage reservoir were discussed. The results shown that it was good way to improve the efficiency of the system by increasing the thermal conductivity of phase change materials and selecting a relatively larger latent heat of fusion. Due to the relatively lower thermal conductivity of phase change materials, different metal foams were composited with the phase change materials in order to improve the heat transfer from the metal hydride bed to the phase change materials and the hydrogen storage efficiency. The effect of aluminium foam on the metal hydride reservoir was studied and validated. The phase change materials composited with copper foam shown better performance than that composited with aluminium foam.     

Numerical analysis on the fluid flow and heat transfer in the channel with V-shaped wavy lower plate

In the present study, the numerical analysis on the heat transfer and flow developments in the channel with one-side corrugated plate under constant heat flux conditions is presented. The corrugated plate with the corrugated tile angles of 40° is simulated with the channel height of 7.5 mm. The flow and heat transfer developments are simulated by the k-ε standard turbulent model. A finite volume method with the structured uniform grid system is employed for solving the model. Effects of relevant parameters on the heat transfer and flow developments are considered. Breaking and destabilizing in the thermal boundary layer are promoted as fluid flowing through the corrugated surface. Therefore, the corrugated surface has a significant effect on the enhancement of heat transfer.     

Modelling and experimental studies on a mixed-mode natural convection solar crop-dryer

A mathematical model for drying agricultural products in a mixed-mode natural convection solar crop dryer (MNCSCD) using a single-pass double-duct solar air-heater (SPDDSAH) is presented. The model was developed in parallel with experimental work. The model comprises the air-heating process model, the drying model and the technical performance criteria model. The governing equations of the drying air temperature and humidity ratio; the material temperature and its moisture content; and performance criteria indicators are derived. The model requires the solution of a number of interrelated non-linear equations and a set of simultaneous differential equations. Results from experimental studies used for generating the required experimental data for validating the model are presented. Results of simulation runs using the model are presented and compared with the experimental data. It is shown that the model can predict the performance of the MNCSCD fairly accurately and therefore can be used as a design tool for prototype development.     

Transient response of plate heat exchangers considering effect of flow maldistribution

Plate heat exchangers have been playing important role in the power and process industries in the recent past. Hence, it is important to develop simulation strategies for plate heat exchangers accurately. This analysis represents the dynamic behaviour of the single pass plate heat exchangers, considering flow maldistribution from port to channel. In addition to maldistribution the fluid axial dispersion is used to characterise the back mixing and other deviations from plug flow. Due to unequal distribution of the fluid, the velocity of the fluid varies from channel to channel and hence the heat transfer coefficient variation is also taken into consideration. Solutions to the governing equations have been obtained using the method of Laplace transform followed by numerical inversion from frequency domain. The results are presented on the effects of flow maldistribution and conventional heat exchanger parameters on the temperature transients of both U-type and Z-type configurations. It is found that the effect of flow maldistribution is significant and it deteriorates the thermal performance as well as the characteristic features of the dynamic response of the heat exchanger. In contrast to the previous studies, here the axial dispersion describes the inchannel back mixing alone, not maldistribution, which is physically more appropriate. Present method is an efficient and consistent way of describing maldistribution and back mixing effects on the transient response of plate heat exchangers using an analytical method without performing intensive computation by complete numerical simulation.     

Improved flow pattern map for accurate prediction of the heat transfer coefficients during condensation of R-134a in smooth horizontal tubes and within the low-mass flux range

This paper presents an improved flow pattern map for predicting the heat transfer coefficients during condensation of R-134a inside a smooth horizontal tube. Experimental tests were conducted over the low-mass flux range of 75–300 kg/m² s, at a nominal saturation temperature of 40 °C, and with the test section vapour qualities ranging from 0.76 down to 0.03. This represents points within the annular, intermittent and stratified flow regimes. The results were used to modify the Thome–El Hajal flow pattern map to include a transition region between the stratified-wavy and annular or intermittent flow regimes. The revised flow pattern-based heat transfer correlation predicted the experimental data to a mean deviation of less than 6%.     

MHD-conjugate heat transfer analysis for a vertical flat plate in presence of viscous dissipation and heat generation

In this paper, the effects of magnetic field, viscous dissipation and heat generation on natural convection flow of an incompressible, viscous and electrically conducting fluid along a vertical flat plate in the presence of conduction are investigated. Numerical solutions for the governing momentum and energy equations are given. A discussion is provided for the effects of magnetic parameter, viscous dissipation parameter and heat generation parameter on two-dimensional flow. Detailed analysis of the velocity profile, temperature distribution, skin friction, rate of heat transfer and the surface temperature distribution are shown graphically.     

Experimental investigation of condensation of hydrocarbon refrigerants (R600a) in a horizontal smooth tube

In this study, the experimental results of the condensing heat transfer coefficients of R600a, a hydrocarbon refrigerant, in a horizontal smooth copper tube with an inner diameter of 4 mm and outer diameter of 6 mm are presented at different vapor quality and different mass fluxes during condensation under annular flow conditions, by adjusting the desired vapor qualities at the test area. A specially-designed sight glass has been fitted to the inlet and outlet of the test tube to identify the flow type by naked eye after the inlet vapor quality of the refrigerant to be fed to the test area during the test is adjusted in the system. Thanks to a new method developed in the measuring system, the condensing heat transfer coefficients could be calculated by measuring the difference value (T_s − T_w) directly from the data collection unit. The experimental findings have shown that the condensing heat transfer coefficients drops down with reduction in vapor quality and the coefficient rises with the increase in the mass flux at constant vapor quality. A correlation has been developed from the data obtained. The condensing heat transfer coefficients obtained from the experimental study were seen to be consistent by ± 20% with the correlations developed by Shah, Travis and Cavallini–Zecchin.     

Heat transfer modelling of a plate radiator for district heating applications

The paper presents a heat transfer model of a plate radiator for district heating applications, developed by the authors. A microcomputer program based on the model was implemented for use in design and simulation of radiator space heating systems. The theoretical model is compared with a model based on experimental data. Results from using the new model to evaluate the accuracy of the German standard DIN 4703 (Q/Q_o = (LMTD/LMTD₀)^1.3) for Icelandic geothermal district heating are presented. Conclusions are drawn on the basis of the results.     

Computational and experimental analysis of high temperature thermal treatment of wood based on ThermoWood technology

Heat treatment of wood at relatively high temperatures (in the range of 180–240 °C) is an effective method to improve the dimensional stability and increase biological durability of wood. In this article, a coupling method is presented for high thermal treatment of a wood based on ThermoWood technology. A three-dimensional mathematical model describing simultaneous unsteady heat and moisture transfer between a gas phase and a solid phase during heat treatment has been developed. The conservation equations for the wood sample are obtained using diffusion equation with variable diffusion coefficients and the 3-dimensional incompressible Reynolds averaged Navier–Stokes equations have been solved for the flow field. The experimental results and model predictions were found to be in good agreement.