Consider a vertical, double-pane window for the conditions prescribed in Problem 9.94. That is, vertical panes at temperature of
Rayleigh number for the onset of convection is
(a) What is the conduction heat flux across the air gap for the optimal spacing
(b) If the glass has an emissivity of
(c) What is the total heat flux if a special, low emissivity coating
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Fundamentals of Heat and Mass Transfer
- 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_forwardA long wire 0.7 mm in diameter with an emissivity of 0.9 is placed in a large quiescent air space at 270 K. If the wire is at 800 K, calculate the net rate of heat loss. Discuss your assumptions.arrow_forwardWhat is the solution to this problem?arrow_forward
- Problem 3 A wet towel (a=75 cm wide, b=1.5 m long) hangs on a cloths line under conditions for which one towel surface receives solar radiation of Qs=900 W/m². Both towel surfaces are exposed to the atmospheric (sky) and ground radiation of Qshy=200 W/m² and Qg=250 W/m², respectively. Under the moderately windy conditions (airflow at a temperature T, = 27 °C, air relative humidity =60%) the convective heat transfer coefficient is o= 20 W/m²-K at both towel surfaces. The wet towel has an emissivity of = 0.96 and the solar absorption coefficient a,=0.65. Determine the towel temperature and the corresponding evaporation rate of moisture from the towel. Assume the air properties at temperature T, = 27 °C.arrow_forward7) Given a window of 20 ft facing east at 40° north latitude, estimate the cooling load due to solar radiation at 4:00 pm for a day in June. Assume the window does not have any interior shading and the shading coefficient is 0.2 (in Btu/hr) (a) 105 (d) 276 (b) 430 (c) 314 (c) 225arrow_forwardA house located in the south of the country was built using windowsdoubles to reduce heat loss to the outside environment on very cold days. Consider that the house has 6 windows 80 cm high and 1.2 m wide, each consisting of 2 layers of glass (kv=1.4 W/(m.oC) 4 mm thick, separated from each other for a spacing of 10 mm filled with stagnant air (ka=0.024 W/(mK), we disregard natural convection effects) On a day when the interior of the house is kept at 22oC, with h=10 W/(m2. K) while the external environment is at 5oC with h=50 W/(m2.K), it is requested:(a) What is the rate of heat dissipation through the windows?(b) What are the temperatures on the glass surfaces inside and in theexterior of the house?(c) Which of the resistances is the limiting in the dissipation process ofheat?(d) Estimate the increase in dissipation rate if we consider that the natural air convection in the space between the windows is characterizedby a convective coefficient h=5 W/(m.K).(e) How the value obtained…arrow_forward
- A long, horizontal, cylindrical steel reactor, 1 m in diameter, has a surface temperature of 300ºC. The emissivity of the steel is 0.6, and the heat transfer coefficient for natural convection is 5 W m−2 K−1 . Heat is lost by convection to the air at 15ºC, and also by radiation to the surroundings, which can be considered to be a black body at 15ºC. a) Calculate the total heat loss per metre length of the reactor, and the proportions lost by convection and radiation b) The reactor is then insulated with a thin layer of insulation material to reduce the total heat loss to one-tenth of its original value. This causes the surface temperature of the steel to rise to 400ºC. The thermal conductivity of the insulation is 0.01 W m−1 K−1 , and its surface emissivity is 0.2. Show that the resulting surface temperature of the insulation is about 89ºC, and calculate the thickness of insulation required, stating any assumptions made. Specifically need help with part barrow_forwardA long, horizontal, cylindrical steel reactor, 1 m in diameter, has a surface temperature of 300ºC. The emissivity of the steel is 0.6, and the heat transfer coefficient for natural convection is 5 W m−2 K−1 . Heat is lost by convection to the air at 15ºC, and also by radiation to the surroundings, which can be considered to be a black body at 15ºC. a) Calculate the total heat loss per metre length of the reactor, and the proportions lost by convection and radiation. b) The reactor is then insulated with a thin layer of insulation material to reduce the total heat loss to one-tenth of its original value. This causes the surface temperature of the steel to rise to 400ºC. The thermal conductivity of the insulation is 0.01 W m−1 K−1 , and its surface emissivity is 0.2. Show that the resulting surface temperature of the insulation is about 89ºC, and calculate the thickness of insulation required, stating any assumptions made. can you solve part b please?arrow_forwardA concrete ball of radius 2.32 m and emissivity 0.620 is heated to 123°C. (a) Convert the given temperature to Kelvin. (Enter your answer to at least three significant figures.) ?K (b) What is the surface area (in m2) of the ball? ?m2 (c) If the ambient temperature is 26.0°C, what net power (in W) does the ball radiate? ?Warrow_forward
- Spacecraft must be cooled via radiative mechanisms, and one means of doing this is by using radiation fins. In many instances, these fins are heated rods that protrude from the spacecraft. Assume that the rod is of length, L; cross-sectional are, Ac; and perimeter, Pā. Its base temperature is T, and the rod has thermal conductivity and emissivity, λ and ɛ, respectively. Set up the differential equation describing the temperature profile within the rod assuming that space is a blackbody at Ts.arrow_forwardThese are different problem from other problems postedarrow_forwardA vertical, double-pane window(square), which is 1 m on a side and has a 25-mm gap filled with atmospheric air, separates quiescent room air at T; = 20 °C from quiescent ambient air at To = -20 °C. Heat loss through the window is 38 W and the heat transfer coefficient between outer glass surface and the outer air is 3.7 W/m²K. Assume the bulk mean temperature between the room air and inner surface of the glass is 15 °C. Radiation heat transfer as well as thermal resistance associated with conduction along glass pane are neglected. The Nusselt number between the air gap of the two glasses of the window. O a. 4.22 O b. 6.21 O C. 5.33 O d.2.16arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning