Heat exchanges convective/radiative

In the case of a given surface temperature, the amount of energy released is determined by the parameters for the convective and radiative heat exchange. As far as convection is concerned, these are the temperatures ol the heat source surface and room air, respectively, and the heat transfer coefficient. The radiative heat exchange is determined by the view factors and the temperatures of the surrounding surfaces. 

For each layer the energy conservation equation is solved. The individual terms are the absorbed radiation in the layer and the radiative and convective heat exchange to the adjacent panes, to the room, or to the exterior. 

The room models implemented in the codes can be distinguished further by how detailed the models of the energy exchange processes are. Simple models use a combined convective-radiative heat exchange. More complex models use separate paths for these effects. Mixed forms also exist. The different models can also be distinguished by how the problem is solved. The energy balance for the zone is calculated in each time step of the simulation. 

The heat transfer term envisions convection to an external surface, and U is an overall heat transfer coefficient. The heat transfer area could be the reactor jacket, coils inside the reactor, cooled baffles, or an external heat exchanger. Other forms of heat transfer or heat generation can be added to this term e.g, mechanical power input from an agitator or radiative heat transfer. The reactor is adiabatic when 7 = 0. 

During the flight of droplets in the spray, the forced convective and radiative heat exchanges with the atomization gas lead to a rapid heat extraction from the droplets. A droplet undergoing cooling and phase change may experience three states (a) fully liquid, (b) semisolid, and (c) fully solid. If the Biot number of a droplet in all three states is smaller than 0.1, the lumped parameter model 1561 can be used for the calculation of droplet temperature. Otherwise, the distributed parameter model 1541 should be used. 

The paper by Haynes and Wepfer (2001) highlights the importance of heat exchange in SOFCs. To achieve an accurate temperature field, they propose a model that takes into account conduction, convection and radiation. The authors show how important the radiative heat exchange is. In particular, on heat transfer from the inner surface of the cell, radiation plays a much more important role than convection, which contributes for about 10%. 

In many applications heat transfer by convection must be considered in addition to radiative heat transfer. This is, for example, the case where a radiator releases heat to a room which is at a lower temperature. Radiative heat exchange takes place between the radiator and the walls of the room, whilst at the same time heat is transferred to the air by convection. These two kinds of heat transfer are parallel to each other and so the heat flow by convection and that by radiation are added together in order to find the total heat exchanged. The heat flux then becomes... 

The thermal system model for radiant-tube continuous furnace involves integration of the mathematical models of the furnace enclosure, the radiant tube, and the load. The furnace enclosure model calculates the heat transfer in the furnace, the furnace gas, and the refractory walls. The radiosity-based zonal method of analysis [159] is used to predict radiation heat exchange in the furnace enclosure. The radiant-tube model simulates the turbulent transport processes, the combustion of fuel and air, and the convective and radiative heat transfer from the combustion products to the tube wall in order to calculate the local radiant-tube wall and gas temperatures [192], Integration of the furnace-enclosure model and the radiant-tube model is achieved using the radiosity method [159]. Only the load model is outlined here. 

If the initial calculations show that the convection heat exchange is not enough to establish the designed combustion intensity, the use of radiant burners becomes decisive. The radiative contribution of the mat is naturally predominant in the power range when flame develops within the mat itself. 

The first boundary condition, Equation 8.9a, implies that we are assuming the wall and bed temperatures are equal at the initial point of contact. Also, the mean free path for the gas at the contact wall is sufficiently large that convective heat exchange by the gas is not in local equilibrium with the conduction through the bed. However, radiative heat transfer can play a vifal role within the penetration layer (Perron and Singh, 1991). As we did for the freeboard, the most practical approach is not only to solve the differential equation but to establish a heat transfer coefficient that can be used for practical calculations. The heat transfer coefficient per unit contact area may be written in terms of the overall heat balance using Newton s law of cooling. 

Heat exchange coefficients Thermal convective coefficient Thermal radiative emissivity... 

Heat exchangers may be installed to cool the off-gas to temperatures suitable for off-gas cleaning. The heat exchangers may be of the radiative or convective type. 

Kakimoto and Liu [10] developed a partly 3D global model that takes into account feasible 3D global modeling with moderate requirements of computer memory and computation time. AH convective and conductive heat transfers, radiative heat exchanges between diffuse surfaces and the Navier-Stokes equations for the melt are all coupled and solved simultaneously by a finite-volume method in a 3D configuration. 

At fuel manifold inlets, gaseous species concentrations are specified as equilibrium compositions of the town gas reformate at 650°C. Steam-to-carbon ratio is kept as 3.06 for this particular steady-state analysis. Both fuel and air gas manifold inlet conditions are summarized in Table 9.5. Mixed convective and radiative heat transfer boundary conditions are applied to the side surfaces of the stack to accurately model the heat exchange with the balance of plant components. Top and bottom surfaces, on the other hand, are assigned with... 

A solar collector uses a smiace that absorbs sunlight, resulting in thermal gain. The hot absorber is in integral contact with a heat-transfer fluid and can thereby deliver domestic hot water, usually via a heat exchanger. To avoid unwanted thermal losses, the backside of the absorber is insulated. Topside radiative and convective losses are minimized by including one or two glazings... 

CS = total-exchange area between gas and surface subscript R indicates allowance for radiatively adiabatic surfaces. h = coefficient of convective heat transfer. 

The heat transfer coefficients am and rad are approximately equal. This infers that free convection to the air and radiative exchange transport almost the same amount of heat. This is not true for forced convection, where m, depending on the flow velocity, is one to two powers of ten larger than the value calculated here. However, ra[Pg.29]

Protective radiation shields are used to reduce the radiative exchange between walls at different temperatures thin foils or sheets made of good reflecting materials are placed between the walls, Fig. 5.66. The spaces between the protective shields are normally evacuated so that heat transfer by convection is prevented. This multi-layer arrangement is used predominantly in cryogenic applications for the insulation of containers for very cold liquified gases. 

Heat transfer or thermal energy exchange occurs if and only if there is a temperature difference. Moreover, thermal energy can only be transferred from a system or substance with a higher temperature to a system or substance with a lower temperature. The phenomenological laws will be discussed here to provide a quantitative relation of a heat flux, as a measure of energy transfer, with a system temperature gradient. Such a relation will be discussed for conductive, convective, and radiative heat transfer mechanisms. 

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