The free convection heat transfer coefficient on a thinhot vertical plate suspended in still air can be determined from observations of the change in plate temperature withtime as it cools. Assuming the plate isisothermal and radiation exchange with its surroundings is negligible, evaluate the convection coefficient atthe instant of time when the plate temperature is 225°Cand the change in plate temperature with time
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Chapter 1 Solutions
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_forwardThe 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_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_forward
- 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_forward1.21 In an experimental set up in a laboratory, a long cylinder with a 5-cm diameter, and an electrical resistance heater inside its entire length is cooled with water flowing crosswise over the cylinder at and a velocity of 0.8 m/s. For these flow conditions, 20 kW/m of power is required to maintain a uniform temperature of at the surface of the cylinder. When water is not available, air at is used with a velocity of 10 m/s to maintain the same surface temperature. However, in this case, the cylinder surface heat dissipation rate is reduced to 400 W/m. Calculate the convection heat transfer coefficients for both water and air, and comment on the reason for the differences in the values.arrow_forwardExhaust gases from a manufacturing plant are being discharged through a 10-m-tall exhaust stack with outer diameter of 1 m. The exhaust gases are discharged at a rate of 0.125 kg/s, while temperature drop between inlet and exit of the exhaust stack is 30°C, and the constant pressure-specific heat of the exhaust gases is 1600 J/kg.K. On a particular calm day, the surrounding quiescent air temperature is 33°C. Solar radiation is incident on the exhaust stack outer surface at a rate of 500 W/m2, and both the emissivity and solar absorptivity of the outer surface are 0.9. Determine the exhaust stack outer surface temperature. Assume the film temperature is 60°C.arrow_forward
- The free convection heat transfer coefficient on a thin hot vertical plate suspended in still air can be determined from observations of the change in plate temperature with time as it cools. Assuming the plate is isothermal and radiation exchange with its surroundings is negligible, evaluate the convection coefficient at the instant of time when the plate temperature is Tplate = 277 °C and the change in plate temperature with time (dT/dt) is −0.022 K/s. The ambient air temperature is Tinf = 28°C and the plate measures 0.7 × 0.7 m with a mass of 5.56 kg and a specific heat of 2,780 J/kg · K.arrow_forwardIn a new residential project, you strongly believethat double-paned windows are ‘better’ than singlepaned windows. Compare the rate of heat loss betweensingle and double-paned windows (1.5 m x 1 m) if thethickness of each pane is (th = 0.4 cm) and (k = 0.9W/m.K). The indoor and outdoor temperatures are 18°C and 2 °C, respectively. Thickness of the air gapbetween the double-paned windows is (th = 1 cm), and(k = 0.022 W/m.K). Image credit: Windowwhirlarrow_forwardPartially-frozen ice cream is being placed in a package before completion of the freezing process. The package has dimensions of 8 cm by 10 cm by 20 cm and is placed in air- blast freezing with convective heat coefficient of 50 W/(m2 K) for freezing. The product temperature is -5°C when placed in the package, and the air temperature is -25°C. The product density is 700 kg/m3, the thermal conductivity (frozen) is 1.2 W/(m K), and the specific heat of the frozen product is 1.9 kJ/(kg K). If the latent heat to be removed during blast freezing is 100 kJ/kg, estimate the freezing time.arrow_forward
- The spherical, thin – walled metallic container is used to store liquid nitrogen at 77 K.The container has a diameter of 0.5 m and is covered with an evacuated insulationsystem composed of silica powder (k = 0.0017 W/m – K). The insulation is 25 mm thick,and its outer surface is exposed to ambient air at 300 K. The latent heat of vaporizationof liquid nitrogen is 2 x 105 J/kg. If the convection coefficient is 20 W/m2 – K over theouter surface. Determine the rate of liquid boil – off of nitrogen per hour. Showexpiration of critical radius of insulation.arrow_forwardThe surface temperature of a 10 m long and a 6-cm-diameter horizontal hot water pipe is 73 ℃. Assume the air temperature to be 27 ℃. Determine the rate of heat loss from the pipe by free convection. Properties of Air are: k = 0.02735 W/m.℃, ν = 1.798*10^-5 m2/s, Pr = 0.7228, β= 1/Tf = 0.003096 K^-1.arrow_forwardIn thermally fully developed flow of a fluid with constant properties, the local convection coefficient is .......................... , ........................ please fill the blank Blank 1( a constant or not a constant) Blank 2 ( dependent on or independent of)arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning