Heat transfer characterization of the solidification process resulting from a spray forming process

Spray forming is achieved by atomizing a liquid metal sheet with an inert gas to form molten droplets, which are then subsequently deposited onto a moving cold substrate. During spray deposition processes, the developing pre-form loses thermal energy through a combination of heat transfer processes. To investigate such issues, a heat transfer model was developed to simulate the pre-form growth. This investigation involves the simultaneous heat transfer in the growing solid and mushy/melt region. The deposition rate is assumed to be continuous rather than discrete. Thus, the heat transfer process describing the growth of the deposit layer is mathematically formulated employing a continuous flow assumption. The influence of the system's controllable parameters such as substrate temperature and velocity, mass deposition rate, the superheat of the impinging metal droplets and environment conditions in the spray chamber on the final deposit and solidification are presented. All the parameters are found to have significant impact on the deposit development.     

Heat and mass transfer in a multi-porous cavity

In this paper, two-dimensional non-linear double diffusive convection in a multi-porous cavity is considered. The Darcy equation, including Brinkman term to account for the viscous effects, is used as the momentum equation. The model consists of two rectangular cavities filled with glass beads having a diameter d₁ of either 5.25 mm (Case 1) or 3.25 mm (Case 2). The smaller cavity is located at the top left corner of the larger one. The larger cavity is filled initially with hot salty fluid while the smaller one contains initially cold fresh fluid. At the initial time, the obstacle between the two cavities was released and the double diffusive phenomena were studied in details. The momentum, solutal, energy and continuity equations are solved numerically using the finite element technique. This transient problem is solved for two different Darcy numbers. For each Darcy number, the influence of the solutal Rayleigh number on double diffusive convection was studied in details. The permeability in the horizontal and vertical direction was assumed identical. A comparison of the intruding force between this case and the open flow case studied by Saghir et al [Int. J. Heat. Mass Transfer, in press] showed that it is inversely proportional to the Darcy number. Finite element modelling results indicate that salinity induces stronger convection than the thermal ones.     

Heat transfer characteristics of spray cooling in a closed loop

A closed loop spray cooling test setup is established for the cooling of high heat flux heat sources. Eight miniature nozzles in a multi-nozzle plate are used to generate a spray array targeting at a 1 × 2 cm² cooling surface. FC-87, FC-72, methanol and water are used as the working fluids. Thermal performance data for the multi-nozzle spray cooling in the confined and closed system are obtained at various operating temperatures, nozzle pressure drops (from 0.69 to 3.10 bar) and heat fluxes. It is exhibited that the spray cooler can reach the critical heat fluxes up to 90 W/cm² with fluorocarbon fluids and 490 W/cm² with methanol. For water, the critical heat flux is higher than 500 W/cm². Air purposely introduced in the spray cooling system with FC-72 fluid has a significant influence on heat transfer characteristics of the spray over the cooling surface.     

Heat and mass transfer analysis of a clay-pot refrigerator

The simple clay-pot refrigerator is ideally suited for preserving vegetarian foods in hot and dry climates. The refrigerator works on the evaporative cooling principle. In this paper, steady-state performance of the refrigerator is analysed using Reynolds flow model of convective heat/mass transfer. For the assumed respiratory cooling load, the preservation temperature is predicted under a variety of ambient temperatures and relative humidities.     

Heat and mass transfer from a horizontal tube of an evaporative heat dissipator

In this investigation, the mass transfer coefficient with simultaneous water and air flow has been determined experimentally. The coefficient has also been calculated theoretically from the convective heat transfer coefficient of air by applying the Lewis relation for an air-water mixture. The ratio of experimental to theoretical mass transfer coefficients has been found to lie between 0.80 to 9.35. A new term ‘evaporative effectiveness’' is defined as the ratio of energy dissipated in evaporative cooling to that in simple water cooling. Its variation is studied and found to range from 0.85 to 1.78. Correlations for design purposes are recommended in this paper.     

Heat transfer to liquid and supercritical helium in superconductingrotors

Cooling designs of superconducting generator rotors are important for maintaining a stable superconducting state of field windings, and it is essential to comprehend the heat transfer characteristics of helium in rotating fields. Experiments were carried out using a large-scale rotating cryostat with a cold rotor diameter of approximately 800 mm. Clear differences among the steady-state heat transfer characteristics of helium were confirmed between low centrifugal acceleration fields and high centrifugal acceleration fields, resulting in the collection of heat transfer data under the supercritical state necessary for cooling designs. The heat transfer characteristics of liquid and supercritical helium under conditions of gravitational and centrifugal acceleration fields are discussed     

Heat and mass transfer model of a Ground Heat Exchanger: theoretical development

A new approach to the simulation of a horizontal type Ground Heat Exchanger is proposed resulting in a better accuracy and at the same time a reduced computational effort. These results come from the concentration of the computational effort at the locations with the largest temperature and moisture gradients, i.e. the pipe–soil interface. The model takes into account heat and moisture transfer in the soil allowing for more accurate predictions of the soil thermal response to the heat fluxes induced by the GHE operation. This in turn allows for a more accurate prediction of the soil temperature field and the circulating fluid temperature profile. A comparison of the results obtained by using the implicit and explicit methods of solving the set of governing equations is discussed. The implicit method requires partial linearization of the heat and mass transfer equations but results in a considerably shorter simulation time. The explicit formulation allows for the solution of the fully nonlinear set of heat and mass transfer equations at the expense of increased simulation time. The following analysis shows that the difference between the solutions obtained using these two methods is minimal, thus favouring the implicit formulation. Copyright © 1999 John Wiley & Sons, Ltd.     

Heat and mass transfer in a countercurrent moving bed dryer

The objective of this work is to study the heat and mass transfer between air and soybean seeds in a countercurrent moving bed dryer, based on the application of a two-phase model to the drying process. The numerical solution of the model is obtained by using a computational code based on BDF methods (Backwards Differentials Formulas). The experimental data of air humidity and temperature and of seed moisture content and temperature at the dryer outlet are compared to the simulated values, showing a good agreement.     

Heat and mass transfer from a single horizontal tube of an evaporative tubular heat exchanger

In this investigation, water side heat transfer coefficients without air flow from a single horizontal tubes are determined. Mass transfer coefficients are determined with water and air flow from the same tube. The total energy dissipated by inside hot fluid when only water is falling is compared with that when both the air and water flow past the tube. The water side heat transfer coefficient and mass transfer coefficient are given by empirical relations h_w = 6.0(Re_p)^0.18(Re_w)^0.87 and K = 3.5(Re_p)^0.18(Re_a)^0.28 (Re_w)^0.54, respectively. The ratio of energies dissipated with water and air flow and with only water flow increases with Re_w and Re_a and its maximum value is 1.72 in the range of variables used.     

Heat and mass transfer analysis of a coal particle undergoing pyrolysis

Heat and mass transfer are combined with kinetics to analyze a coal particle undergoing pyrolysis. The effects of particle size and surrounding temperature are investigated in terms of heat transfer while the effect of pressure is attributed to the competition of pyrolysis, secondary reaction, and the mass transport within the particle. At a high heat transfer rate, the weight loss is determined by the competition between pyrolysis and a secondary reaction. At a low heating rate, the contribution of internal mass transport becomes more pronounced. The weight loss is then determined by the competition between such internal transport and the deposition reaction.     

Heat and mass transfer in a square microchannel with asymmetric heating

This paper describes the heat and mass transfer in a square microchannel that is heated from one side. This microchannel represents a reaction channel in a microreactor that is used to study the kinetics of the catalytic partial oxidation of methane. The microchannel is contained in a silicon wafer and is covered by a thin silicon sheet. At the top side of this sheet, heating elements are present which mimic the heat that is produced as a result of the exothermic chemical reaction. Correlations for Nusselt and Sherwood numbers as a function of the Graetz number are derived for laminar and plug flow conditions. These correlations describe the heat and mass transport at the covering top sheet of the microchannel as well as at its side and bottom walls. By means of computational fluid dynamic simulations, the laminar flow is studied. To determine an approximate laminar flow Nusselt correlation, the heat transport was solved analytically for plug flow conditions to describe the influence of changes in the thermal boundaries of the system. The laminar flow case is experimentally validated by measuring the actual temperature distribution in a 500 μm square, 3 cm long, microchannel that is covered by a 1 μm and by a 1.9 μm thick silicon sheet with heating elements and temperature sensors on top. The Nusselt and Sherwood correlations can be used to readily quantify the heat and mass transport to support kinetic studies of catalytic reactions in this type of microreactor.