The diameter and surface emissivity of an electricallyheated plate are
(a) Estimate the power needed to maintain a surfacetemperature of 200°C in a room [or which the airand the walls are at 25°C. The coefficient characterizing heat transfer by natural convection depends on the surface temperature and, in units of
(b) Assess the effect of surface temperature on thepower requirement, as well as on the relative contributions of convection and radiation to heat transfer from the surface.
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Fundamentals of Heat and Mass Transfer
- An electrical transmission line of 1.2-cm diameter carries a current of 200 amps and has a resistance of 310-4 ohm per meter of length. If the air around this line is at v, determine the surface temperature on a windy day, assuming a wind blows across the line at 33 km/h.arrow_forwardA horizontal, 3-mm-thick flat-copper plate, 1-m long and 0.5-m wide, is exposed in air at 27C to radiation from the sun. If the total rate of solar radiation absorbed is 300 W and the combined radiation and convection heat transfer coefficients on the upper and lower surfaces are 20 and 15W/m2K, respectively, determine the equilibrium temperature of the plate.arrow_forwardA spherical communications satellite, 2 m in diameter, is placed in orbit around the earth. The satellite generates 1000 W of internal power from a small nuclear generator. If the surface of the satellite has an emittance of 0.3, and is shaded from solar radiation by the earth, estimate its surface temperature.arrow_forward
- 1.13 If the outer air temperature in Problem is –2°C, calculate the convection heat transfer coefficient between the outer surface of the window and the air, assuming radiation is negligible.arrow_forwardDetermine the power requirement of a soldering iron in which the tip is maintained at 400C. The tip is a cylinder 3 mm in diameter and 10 mm long. The surrounding air temperature is 20C, and the average convection heat transfer coefficient over the tip is 20W/m2K. The tip is highly polished initially, giving it a very low emittance.arrow_forwardpnts) Chips of width L _ 15 mm on a side are mounted to a substrate that is installed in an enclosure whose walls and air are maintained at a temperature of Tsur=T∞=25oC. The chips have an emissivity of ε=0.60 and a maximum allowable temperature of Ts=85oC. (a) If heat is rejected from the chips by radiation and natural convection, what is the maximum operating power of each chip? The convection coefficient may be approximated as h=11.7 W/m2K. (b) If a fan is used to maintain airflow through the enclosure and heat transfer is by only forced convection, with h=250 W/m2K, what is the maximum operating power?arrow_forward
- (a) If the cost of producing the hot water is $0.10 per kWh, what is the representative daily costof heat loss from an uninsulated pipe to the air per meter of pipe length? The convectionresistance associated with water flow in the pipe may be neglected.(b) Determine the savings associated with application of a 10-mm-thick coating of urethaneinsulation (k = 0.026 W/m-K) to the outer surface of the pipe.arrow_forwardA circular grill of diameter 0.35 m and emissivity 0.7 is maintained at a constant surface temperature of 130 °C in a horizontal position. What electrical power is required when the room air and surroundings are at 24 °C?arrow_forwardDifferentiate between free and forced convection with examples.arrow_forward
- 3- You have experienced convection cooling if you have ever extended your handout of the window of a moving vehicle or into a flowing water stream. With the surface of your hand at a temperature of 30 oC, determine the convection heat flux for a) a vehicle speed of 35 km/h in air at -5 oC with convection coefficient of 40 W/m2 .K and b) a velocity of 0.2 m/s in a water stream at 10 oC with convection coefficient of 900 W/m2.K. Which condition would feel colder? Contrast theseresults with a heat loss of approximately 30 W/m2 under normal room conditions.arrow_forwardQuestion 1:A glass window of width W = 1 m and height H = 2 m is 5 mm thick and has a thermal conductivity of kg=1.4 W/m.K. If the inner and outer surface temperatures of the glass are 15oC and -20oC, respectively, ona cold winter day, what is the rate of heat loss throughthe glass? To reduce heat loss through windows, it iscustomary to use a double pane construction in whichadjoining panes are separated by an air space. If thespacing is 10 mm and the glass surfaces in contact withthe air have temperatures of (last two digit of your Registration number) oC and -15oC, what isthe rate of heat loss from a 1 m x 2 m window? Thethermal conductivity of air is ka=0.024 W/m.K.arrow_forwardQuestion 1: A glass window of width W = 1 m and height H = 2 m is 5 mm thick and has a thermal conductivity of kg =1.4 W/m.K. If the inner and outer surface temperatures of the glass are 15 oC and -20 oC, respectively, on a cold winter day, what is the rate of heat loss through the glass? To reduce heat loss through windows, it is customary to use a double pane construction in which adjoining panes are separated by an air space. If the spacing is 10 mm and the glass surfaces in contact with the air have temperatures of 20 oC and -15 oC, what is the rate of heat loss from a 1 m x 2 m window? The thermal conductivity of air is ka = 0.024 W/m.K.arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning