Thermal radiation heat transfer coefficient

C Consider a surface of area A at which the convection and radiation heat transfer coefficients are and respectively. Explain how you would determine (a) the single equivalent heat transfer coefficient, and (b) the equivalent thermal resistance. Assume the medium and the surrounding surfaces are at the same temperature. 

Note that the operative temperature will be the arithmetic average of the ambient and surrounding surface temperatures when the convection and radiation heat transfer coefficients are equal to each other. Another environmental index used in thermal comfort analysis is the effective temperature, which combines the effects of temperature and humidity. Two environments with the same effective temperature evokes the same thermal response in people even though they are at different temperatures and humidities. 

Design Methods for Calciners In indirect-heated calciners, heat transfer is primarily by radiation from the cyhnder wall to the solids bed. The thermal efficiency ranges from 30 to 65 percent. By utilization of the furnace exhaust gases for preheated combustion air, steam produc tion, or heat for other process steps, the thermal efficiency can be increased considerably. The limiting factors in heat transmission he in the conductivity and radiation constants of the shell metal and solids bed. If the characteristics of these are known, equipment may be accurately sized by employing the Stefan-Boltzmann radiation equation. Apparent heat-transfer coefficients will range from 17 J/(m s K) in low-temperature operations to 8.5 J/(m s K) in high-temperature processes. 

ORNL small-break LOCA tests Experimental investigation of heat transfer and reflood analysis was made under conditions similar to those expected in a small-break LOCA. These tests were performed in a large, high-pressure, electrically heated test loop of the ORNL Thermal Hydraulic Test Facility. The analysis utilized a heat transfer model that accounts for forced convection and thermal radiation to steam. The results consist of a high-pressure, high-temperature database of experimental heat transfer coefficients and local fluid conditions. 

The parametric effect of bed temperature is expected to be reflected through higher thermal conductivity of gas and higher thermal radiation fluxes at higher temperatures. Basu and Nag (1996) show the combined effect (Fig. 23) which plots heat transfer coefficients as a function of bed temperature for data from four different sources. It is seen that for particles of approximately the same diameter, at a constant suspension density (solid concentration), the heat transfer coefficient increases by almost 300% as the bed temperatures increase from 600°C to 900°C. 

All correlations based on ambient temperature data where thermal radiation is negligible should be considered to represent only the convective heat transfer coefficient hc. 

Here, k is the effective thermal conductivity, A is the effective contact area between the adjacent cells, l is the characteristic conduction length scale, hconv is the convection heat transfer coefficient, Aext external surface area of the cell exposed to the ambient air, 7 x is the ambient temperature and P is the cell power. The characteristic conduction length is calculated as the volume of the bipolar plate divided by the cell normal area. Factor /3 is an empirical constant which is the ratio of the heat generated to the power produced by the cell, i.e. (1 - rj), rj being the efficiency. When radiation is considered, should be included in Equation (5.64). The heat transfer relationships between the gas channels and the solid regions are given by ... 

Industrial reactors are thermally insulated for safety reasons (hot surfaces) and for economical reasons (heat losses). Nevertheless, at higher temperatures, heat losses may become important. Their calculation may become tedious, since heat losses are often due to a combination of losses by radiation and by natural convection. If an estimation is required, a simplified expression using a global overall heat transfer coefficient (a) may be useful ... 

The contents of a reaction vessel are heated by means of steam at 393 K supplied to a heating coil which is totally immersed in the liquid. When the vessel has a layer of lagging 50 mm thick on its outer surfaces, it takes one hour to heat the liquid from 293 to 373 K. How long will it take if the thickness of lagging is doubled Outside temperature = 293 K. Thermal conductivity of lagging = 0.05 W/mK. Coefficient for heat loss by radiation and convection from outside surface of vessel = 10 W/m2 K. Outside area of vessel = 8 m2. Coil area = 0.2 m2. Overall heat transfer coefficient for steam coil = 300 W/m2 K. 

The static contribution l. , incorporates heat transfer by conduction and radiation in the fluid present in the pores, conduction through particles, at the particle contact points and through stagnant fluid zones in the particles, and radiation from particle to particle. Figure 19-20 compares various literature correlations for the effective thermal conductivity and wall heat-transfer coefficient in fixed beds [Yagi and Kunii, AlC hE J. 3 373(1957)]. 

Extensive experimental determinations of overall heat transfer coefficients over packed reactor tubes suitable for selective oxidation are presented. The scope of the experiments covers the effects of tube diameter, coolant temperature, air mass velocity, packing size, shape and thermal conductivity. Various predictive models of heat transfer in packed beds are tested with the data. The best results (to within 10%) are obtained from a recently developed two-phase continuum model, incorporating combined conduction, convection and radiation, the latter being found to be significant under commercial operating conditions. 

The preceding discussion in this subsection does not include the effects of thermal radiation. If the radiation effects can be linearized (that is, assuming small temperature differences), the heat transfer coefficient at the outer surface, h0, takes the form... 

From time to time we have mentioned that thermal conductivities of materials vary with temperature however, over a temperature range of 100 to 200°C the variation is not great (on the order of 5 to 10 percent) and we are justified in assuming constant values to simplify problem solutions. Convection and radiation boundary conditions are particularly notorious for their nonconstant behavior. Even worse is the fact that for many practical problems the basic uncertainty in our knowledge of convection heat-transfer coefficients may not be better than 20 percent. Uncertainties of surface-radiation properties of 10 percent are not unusual at all. For example, a highly polished aluminum plate, if allowed to oxidize heavily, will absorb as much as 300 percent more radiation than when it was polished. 

The inner and outer surfaces of a 25-cm-thick wall in summer are at 27"C and 44 C, respectively. The outer surface of the wall exchanges heal by radiation with surrounding surfaces at 40°C, and convection svith ambient air also at 40 C with a convection heat transfer coefficient of 8 W/m - °C. Solar radiation is incident on the surface at a rate of 150 W/m If both the emissivity and the solar absorptivity of the outer surface are 0.8, determine the effective thermal conductivity of the wall. 

A spherical metal ball of radius is heated in an oven to a temperature of T, throughout and is then taken out of the oven and allowed to cool in ambient air at T by convection and radiation. The emissivily of the outer surface of (he cylinder is c, and the temperature of the surrounding surfaces is The average convection heat transfer coefficient is estimated to be II Assuming variable thermal conductivity and transient one-diiiiensional lieat transfer, express the mathematical formulation (the differential equation and the boundary... 

A 1000-W iron is left on Ihe iron board with its base exposed to ambient air at 26°C. The base plate of the iron has a thickness ofL= 0.5 cm, base area of A = 150 cm, and thermal conductivity of k = 18 W/m C.The inner surface of the base platens subjected to uniform heat flux generated by the resistance heaters inside. The outer surface of the base plate whose emissivity is k = 0.7, loses he4l by convection to ambient air with an average heat transfer coefficient of A = 30 W/pi °C as well as by radiation to the surrounding... 

A solar heat flux q, is incident on a sidewalk whose thermal conductivity is k, solar absorptivity is a and convective heat transfer coefficient is h. TaWng the positive x direction to be towards the sky and disregarding radiation exchange with the surroundings surfaces, the correct boundary condition for this sidewalk surface is... 

We stait this chapter with one-dimensional steady heat conduction in a plane wall, a cylinder, and a sphere, and develop relations for thennal resistances in these geometries. We also develop thermal resistance relations for convection and radiation conditions at the boundaries. Wc apply this concept to heat conduction problems in multilayer plane wails, cylinders, and spheres and generalize it to systems that involve heat transfer in two or three dimensions. We also discuss the thermal contact resislance and the overall heat transfer coefficient and develop relations for the critical radius of insulation for a cylinder and a sphere. Finally, we discuss steady heat transfer from finned surfaces and some complex geometries commonly encountered in practice through the use of conduction shape factors. 

Consider a 0.8-m-high and 1.5-m-wide glass vyindow with a thickness of 8 mm I and a thermal conductivity of /r = 0.78 W/m K, Determine the steady rate of heat transfer through this glass v/indow and the temperature of Its inner surface for a day during which the room is maintained at 20 C while the temperature df the outdoors Is - lO C. Take the heat transfer coefficients on the inner and outer surfaces of the window to be /), = 10 W/m C and fr = 40 W/m "C, which includes the effects of radiation. 

Assumptions 1 Heat transfer is steady since there is no indication of any change with time. 2 Heat transfer is one-dimensional since there is thermal symmetry about the centerline and no variation in the axial direction. 3 Thermal conductivities are constant. 4 The thermal contact resistance at the interface is negligible. 5 Heat transfer coefficient incorporates the radiation effects, if any.. . . .. 

Assumptions 1 Steady operating conditions exist. 2 The heat transfer coefficient is uniform over the entire fin surfaces. 3 Thermal conductivity is constant. 4 Heat transfer by radiation is negligible. 

Assumptions I The body can be modeled as a 30-cm-diameter, 1.70-m-long cylinder. 2 The thermal properties of the body and the heat transfer coefficient are7constpnt. 3 The radiation effects are negligible. 4 The person was healthyO) when he or she died with a body temperature of 37°C. 

Consider a plane wall of thickness 2L, a long cylinder of radius r , and a sphere of radius r, initially at a nnifonn temperature T,-, as shown in Fig. 4—11. At time t = 0, each geometry is placed in a large medium that is at a constant temperature T and kepi in that medium for t > 0. Heat transfer lakes place between these bodies and their environments by convection with a uniform and constant heal transfer coefficient A. Note that all three ca.ses possess geometric and thermal symmetry the plane wall is symmetric about its center plane (,v = 0), the cylinder is symmetric about its centerline (r = 0), and the sphere is symmetric about its center point (r = 0). We neglect radiation heat transfer between these bodies and their surrounding surfaces, or incorporate the radiation effect into the convection heat transfer coefficient A. 

Even in. simple geometries, heat transfer problems cannot be. solved analytically if the thermal conditions are not sufficiently simple. For example, the consideration of the variation of thermal conductivity with temperature, the variation of the heat transfer coefficient over the surface, or the radiation heat transfer on the surfaces can make it impossible to obtain au analytical. solution. Therefore, analytical solutions are limited to problems that are simple or can be simplified with rea.sonable approximations. 

S-82 Consider transient one-dimensional heat conduction in a pin fin of constant diameter O with constant thermal conductivity. The fin is losing heat by convection to the ambient air at r. with a heat transfer coefficient of li and by radiation to the surrounding surfaces at an average temperature of The nodal network of the fin consists of nodes 0 (at the base), 1 (in the middle), and 2 (at the fin tip) with a iinifonn nodal spacing of A.Y. Using the energy balance approach, obtain the explicit finite difference formulation of (his problem for the case of a specified temperature at the fin base and negligible heat transfer at the fin tip. 

Consider a l.2-m-high and 2-m-wide glass window with a thickness of 6 nun, thermal conductivity k = 0.78 W/m C, and emissivity e = 0.9. The room and the walls that face the window are maintained at 25°C, and the average temperature of the inner surface of the window is measured to be 5°C. If the temperature of Ihe outdoors is -5 C, determine (a) the convection heat transfer coefficient on Ihe inner surface of the window, (b) the rate of total heat transfer through the window, and (c) the combined natural convection and radiation beat transfer coefficient on the outer... 

Determine the CZ-faclors for the center-of-glass section of a double-pane window and a triple-pane window, fhe heat transfer coefficients on the inside and outside surfaces are 6 and 25 W/ni °C, respectively. The thickness of the air layer is 1.5 cm and there are two such air layers in tiiple-pane window. The Nu.sselt number across an air layer is estimated to be 1.2- Take the thermal conductivity of air to be 0.025 W/m C and neglect the thermal resistance of glass sheets. Also, assume that the effect of radiation through the air space is of the same magnitude as the convection. 

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