Concept explainers
A common arrangement for heating a large surface area is to move warm air through rectangular ducts below the surface. The ducts are square and located midway between the top and bottom surfaces that are exposed to room air and insulated, respectively.
For the condition when the floor and duct temperatures are 30 and 80°C, respectively, and the thermal conductivity of concrete is
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Fundamentals of Heat and Mass Transfer
- The air-conditioning system in a Chevrolet van for use in desert climates is to be sized. The system is to maintain an interior temperature of 20C when the van travels at 100 km/h through dry air at 30C at night. If the top of the van is idealized as a flat plate 6 m long and 2 m wide and the sides as flat plates 3 m tall and 6 m long, estimate the rate at which heat must be removed from the interior to maintain the specifiedarrow_forward1.1 On a cold winter day, the outer surface of a 0.2-m-thick concrete wall of a warehouse is exposed to temperature of –5°C, while the inner surface is kept at 20°C. The thermal conductivity of the concrete is 1.2 W/m K. Determine the heat loss through the wall, which is 10-m long and 3-m high. Problem 1.1arrow_forward2.42 A circumferential fin of rectangular cross section, 3.7-cm OD and 0.3 cm thick, surrounds a 2.5-cm- diameter tube as shown below. The fin is constructed of mild steel. Air blowing over the fin produces a heat transfer coefficient of K. If the temperatures of the base of the fin and the air are and , respectively, calculate the heat transfer rate from the fin.arrow_forward
- The heat transfer coefficient for a gas flowing over a thin float plate 3-m long and 0.3-m wide varies with distance from the leading edge according to hc(x)=10x1/4Wm2K If the plate temperature is 170C and the gas temperature is 30C, calculate (a) the average heat transfer coefficient, (b) the rate of heat transfer between the plate and the gas, and (c) the local heat flux 2 m from the leading edge. Problem 1.18arrow_forward1.4 To measure thermal conductivity, two similar 1-cm-thick specimens are placed in the apparatus shown in the accompanying sketch. Electric current is supplied to the guard heater, and a wattmeter shows that the power dissipation is 10 W. Thermocouples attached to the warmer and to the cooler surfaces show temperatures of 322 and 300 K, respectively. Calculate the thermal conductivity of the material at the mean temperature in W/m K. Problem 1.4arrow_forwardQuestion 3: A house has a composite wall of wood, fiberglass insulation, and plaster board, as indicated in the sketch. On a cold winter day the convection heat transfer coefficients are ho 60 W/m2 K and hi 30 W/m2 K. The total wall surface area is 350 m2. (a)Determine the total heat loss through the wall. (b) If the wind were blowing violently, raising ho to 300 W/m2 K, determine the percentage increase in the heat loss.arrow_forward
- Steam at a temperature of 723 K is flowing through a pipe with an outer diameter of 100 mm. The pipe is insulated with 30-mm thick mineral wool, the average thermal conductivity of which is 0.065 W/m.K. Determine the outside surface temperature of mineral wool if the ambient air temperature is 300 Assume the hot-surface temperature of the mineral wool is the same as that of the flowing steam and the value of the surface coefficient of the insulating material is 33 W/m2.K.arrow_forwardChips 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 powerarrow_forwardA 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 9oC 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
- Calculate the heat loss per square foot of wall surface for a wallmade of 1-in. of plaster and 9-in. of common brick. The air velocityover the surface of the brick is 15 mph; the outside air temperature is40 °F, and the surface temperature of the plaster facing the room is 70°F.sarrow_forwardFinned heat sinks are used to keep computer chips in an acceptable temperature range. The heat sink base is 53 mm x 57 mm, and the maximum allowable chip temperature is 75 C. An array of 54 aluminum fins (k = 237 W/m*K) which have a square cross-section (3 mm x 3 mm) are attached to the heat sink. The length of each fin is 30 mm. The cooling air supplied to the heat sink is 25 C. If the convection heat transfer coefficient is 125 W/m2*K, what is the heat transfer rate (in W) from the heat sink, assuming adiabatic fin tips? (Hint: heat transfer does not solely occur from the fins.)arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning