Heat transfer coefficients radiation

K, i = radiation heat transfer coefficient from salt water surface to cover of still, B.t.u./hr., sq. ft., ° F. 

Since ratiation heat-transfer problems are often very closely associated with convection problems, and the total heat transfer by both convection and radiation is often the objective of an analysis, it is worthwhile to put both processes on a common basis by defining a radiation heat-transfer coefficient ft, as... 

In many instances the convection heat-transfer coefficient is not strongly dependent on temperature. However, this is not so with the radiation heat-transfer coefficient. The value of ft, corresponding to Eq. (8-43), could be calculated from... 

A thin metal plate is insulated on the back and exposed to solar radiation at the front surface (Fig. 1-44). The exposed surface of the plate has an absorptivity of 0.6 for solar radiation. If solar radiation is incident on the plate at a rate of 700 W/m and the surrounding air temperature is 25°C, determine the surface temperature of the plate when the heat loss by convection and radiation equals the solar energy absorbed by the plate. Assume the combined convection and radiation heat transfer coefficient to be 50 W/m °C. 

A lO-cm-high and 20-cm-widc circuit board houses on its surface 100 closely spaced chips, each generating heat al a rate of 0.08 W and transfeiiiiig it by coiivecibii and radiation to the surrounding medium at 40°C, Heat transfer from the back surface of the board is negligible. If the combined convection and radiation heat transfer coefficient on the surface of the board is 22 W/m - "C, the average. surface temperature of the chips is... 

Four identical power transistors with aluminum casing are attached on one side of a 1-cm-thick 20-cm x 20-cm square copper plate (k = 386. W/m - X) by screw that exert an average pressure of. 6 MPa (Fig. 3-18). The base area of each transistor is 8 cm, and each transistor is placed at the center of a. 10-cm X 10-cm quarter section of the plate. The interface roughness is estimated to be about 1.5 /rm. All transistors are covered by a tfdck Plexiglas layer, which is a poor conductor of heat, and thus all the heat generated at the junction of the transistor must be dissipated to the ambierrt at 20X through the back surface of the copper plate. The combined convection/radiation heat transfer coefficient at the back surface can be taken to be 25 W/m C. If the case temperature of the... 

Also, the radiation heat transfer coefficient is given by... 

We note that must be between 0°C and 22°C. but it must be closer to 0°C, since the heat transfer coefficient inside the tank is much larger. Taking Ti = 5°C = 278 K. the radiation heat transfer coefficient is determined to be... 

Heat transfer tiu-ough a wall or roof section is also affected by the convection and radiation heat transfer coefficients at the exposed surface.s. The effects of convection and radiation on the inner and outer surfaces of walls and roofs are usually combined into the combined convection and radiation heat transfer coefficients (also called surface conductances) lij and /i ,... 

Combined convection and radiation heat transfer coefficients at window, wall, or roof surfaces (from ASHRAE Handbook of Fundamentals. 

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. 

A cylindrical resistor elefneot on a circuit board dissipates 0.15 W of power in an environment at 4ff C. The resistor is i. 2 cn long, and has a diameter of 0.3 cm. Assuming heal to be transferred uniformly front all surfaces, determine (a) the amount of heat this resistor dissipates during a 24-h period (b) the heat flux on the surface of the resistor, in W/m and (if) the surface temperature of the resistor for a combined convection and radiation heat transfer coefficient of 9 W/m C. 

A 2-m X t,5-in section of wall of an industrial furnace burning natural gas is not insulated, and the temperatuce at the outer surface of this section is iiieasuied to be 80°C. The tem-peralure of the furnace room is 30 C, and the combined convection and radiation heat transfer coefficient at the surface of the outer furnace is 10 W/m °C, It is proposed to insulate this section of tiie furnace wall with glass wool insulation (A = 0,038 W/m -" C) in order to reduce the heat loss by 90 percent, Assuming the outer surface temperature of the metal section still remains at about 80°C, determine the thickness of the insulation that needs lobe used. 

Consider a cold aluminum canned drink that is initially at a uniform temperature of 4°C. The can is 12.5 cm high and has a diameter of 6 cm. If the combined convection/radiation heat transfer coefficient between the can and the surrounding air at 25°C is 10 W/m - C, determine how long it will take for the average temperature of the drink to rise to IS C. 

A 5-mm-diameler spherical ball at 50 C is covered by a 1-mm-lhick plastic insulation (k = 0.13 W/m - °C). The ball is exposed to a medium at I5 C, with a combined convection and radiation heat transfer coefficient of 20 W/m - C. Determine if the plastic insulation on the ball will help or hurt heat transfer from the ball. 

The plumbing system of a house involves a 0.5-m section of a plastic pipe (fc = 0.16 VV/m °C) of inner diameter 2 cm and outer diameter 2.4 cm exposed to tl e ambient air. During a cold and windy niglit, the ambient air temperature remains at about —5°C for a period of 14 h. The combined convection and radiation heat transfer coefficient on the outer surface of the pipe is estimated to be 40 W/ni C, and the heat of fusion of water is 333.7 kJ/kg. Assuming the pipe to contain stationary water initially at 0°C, deienninc if the water in that section of the pipe will completely freeze that night. 

Cold conditioned air ai I2°C is flowing inside a 1.5-cm-lhick square aluminum (A = 237 W/m "C) duct of inner cross section 22 cm X 22 cm at a mass flow rale of 0.8 kg/s. 1 he duct is exposed to air at 33°C with a combined convection-radiation heat transfer coefficient of 13 W/m - °C. Tlie convection heat transfer coefficient at the inner surface is 75 W/m C, If the air temperature in the duct should not increase by more than i °C determine the maximuin length of the duct. 

Inli production facility, large brass plates of 4-cm thickness that are initially at a uniform temperature of 20 C are heated by passing them through an oven that is maintained at 500°C (Fig. 4-22). The plates remain in the oven for a period of 7 min. Taking the combined convection and radiation heat transfer coefficient to be / = 120 W/m °C, determine the surface temperature of the plates when they come out of the oven. 

Combined convection and radiation heat transfer coefficient hi at the inner surface of a vertical glass under still air conditions... 

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. 

Analysis The body loses heat in sensible and latent forms, and the sensible heat consists of convection and radiation heat transfer. At lovy air velocities, the convection heat transfer coefficient for a standing man is given in Table 13-5 to be 4.0 W/m - C. The radiation heat transfer coefficient at typical indoor conditions is 4.7 W/m "C. Therefore, the surface heat transfer coefficient for a standing person for combined convection and radiation is... 

HTR.I2 RADIATION HEAT-TRANSFER COEFFICIENT 107 The emissive power, E, of a real surfaee is... 

It is often convenient to express the radiation heat transfer in a Newton s law of cooling form. In this case, a radiation heat-transfer coefficient, h, is defined by equating the Newton s law of cooling rate to the radiation heat-transfer rate, i.e.,... 

Radiative heat transfer is often coupled wim convective heat transfer to form me boundary conditions for conduction problems. A radiation heat transfer coefficient may be defined by 7rad = A rad (T) ), where /trad = iO(7i + )(7i + Tsur) is a strong function of me temperatures... 

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