The ratio of heat flow Q1/Q2 from two walls of same thicknessbe selected)having their thermal resistances as Rth1=2 Rth2 will be:40% of an incident radiant energy on the surface of a thermallytransparent body is reflected back. If the transmissivity of the bodyQISelect the correct answer:1.A. 0.125B. 0.5С.0.25D. 22.The absorptivity of a blackbody is equal to:В.0.5A. 1C.D.None of themzero3.be 0.15, then the emissivity of surface is:В. О.55A.0.45С.D. None of them0.404.Unsteady state heat flow means,A. Negligible flow of heatB. No difference of temperature betweenthe bodiesC.Heat flow independent ofD. None of themtime5.The boundary conditions of the infinite length fin are:dT- kAА.В.and T-To- kA(= 0 and T=T00at x=0at x=LD.None of themT=0 at x-0 and q=0 at x-LTwo concentric cylinders are radiated heat to each other, the innercylinder (1) is (10cm) diameter, the outer cylinder (2) is (30cm)diameter, the shape factor F21 is equal to:А. 0.125С. 2Plane wall with heat generation, at x-0 (1=T1), at x-L (T=T2) and(T1 T2), The temperature distribution inside the wall is:С.6.В. 0.33D. None of them7.В.PlaneА.PlanewallwalD.PaneС.PlanewallPage 1 of 3

1)Given data Q1 be the heat flow of wall 1 Q2 be the heat flow of wall 2

be selected) transparent body is reflected back. If the transmissivity of the body having their thermal resistances as Rth1=2 Rth2 will be: 40% of an incident radiant energy on the surface of a thermally 1. The ratio of heat flow Q1/Q2 from two walls of same thickness QI Select the correct answer: В. А. 0.125 0.5 D. 2 С. 0.25 The absorptivity of a blackbody is equal to: A. 1 С. 2. В. 0.5 D. None of them zero 3. be 0.15, then the emissivity of surface is: A. 0.45 В. О.55 D. None of them С. 0.40 4. Unsteady state heat flow means, A. Negligible flow of heat B. No difference of temperature between the bodies С. Heat flow independent of D. None of them time 5. The boundary conditions of the infinite length fin are: А. KA( =0 В. -kA(- and T=To =0 and T-Too %3D %3D at x=L at x-0 D. None of them С. T=0 at x-0 and q=0 at x-L dintod hoat to oach other the inner

be selected) transparent body is reflected back. If the transmissivity of the body having their thermal resistances as Rth1=2 Rth2 will be: 40% of an incident radiant energy on the surface of a thermally 1. The ratio of heat flow Q1/Q2 from two walls of same thickness QI Select the correct answer: В. А. 0.125 0.5 D. 2 С. 0.25 The absorptivity of a blackbody is equal to: A. 1 С. 2. В. 0.5 D. None of them zero 3. be 0.15, then the emissivity of surface is: A. 0.45 В. О.55 D. None of them С. 0.40 4. Unsteady state heat flow means, A. Negligible flow of heat B. No difference of temperature between the bodies С. Heat flow independent of D. None of them time 5. The boundary conditions of the infinite length fin are: А. KA( =0 В. -kA(- and T=To =0 and T-Too %3D %3D at x=L at x-0 D. None of them С. T=0 at x-0 and q=0 at x-L dintod hoat to oach other the inner

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter7: Forced Convection Inside Tubes And Ducts

Section: Chapter Questions

Problem 7.29P

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2.45 Heat is transferred from water to air through a brass wall . The addition of rectangular brass fins, 0.08 cm thick and 2.5 cm long, spaced 1.25 cm apart, is contemplated. Assuming a water-side heat transfer coefficient of and an airside heat transfer coefficient of , compare the gain in heat transfer rate achieved by adding fins to (a) the water side, (b) the air side, and (c) both sides. (Neglect temperature drop through the wall.)
1.10 A heat flux meter at the outer (cold) wall of a concrete building indicates that the heat loss through a wall of 10-cm thickness is . If a thermocouple at the inner surface of the wall indicates a temperature of 22°C while another at the outer surface shows 6°C, calculate the thermal conductivity of the concrete and compare your result with the value in Appendix 2, Table 11.
1.37 Mild steel nails were driven through a solid wood wall consisting of two layers, each 2.5-cm thick, for reinforcement. If the total cross-sectional area of the nails is 0.5% of the wall area, determine the unit thermal conductance of the composite wall and the percent of the total heat flow that passes through the nails when the temperature difference across the wall is 25°C. Neglect contact resistance between the wood layers.
2.38 The addition of aluminum fins has been suggested to increase the rate of heat dissipation from one side of an electronic device 1 m wide and 1 m tall. The fins are to be rectangular in cross section, 2.5 cm long and 0.25 cm thick, as shown in the figure. There are to be 100 fins per meter. The convection heat transfer coefficient, both for the wall and the fins, is estimated to be K. With this information determine the percent increase in the rate of heat transfer of the finned wall compared to the bare wall.
1.4 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 1.4
Using Table 1.4 as a guide, prepare a similar table showing the orders of magnitude of the thermal resistances of a unit area for convection between a surface and various fluids.
2.2 A small dam, which is idealized by a large slab 1.2 m thick, is to be completely poured in a short Period of time. The hydration of the concrete results in the equivalent of a distributed source of constant strength of 100 W/m3. If both dam surfaces are at 16°C, determine the maximum temperature to which the concrete will be subjected, assuming steady-state conditions. The thermal conductivity of the wet concrete can be taken as 0.84 W/m K.
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.
An electronic device that internally generates 600 mW of heat has a maximum permissible operating temperature of 70C. It is to be cooled in 25C air by attaching aluminum fins with a total surface area of 12cm2. The convection heat transfer coefficient between the fins and the air is 20W/m2K. Estimate the operating temperature when the fins are attached in such a way that (a) there is a contact resistance of approximately 50 K/W between the surface of the device and the fin array and (b) there is no contact resistance (in this case, the construction of the device is more expensive). Comment on the design options.
3.10 A spherical shell satellite (3-m-OD, 1.25-cm-thick stainless steel walls) re-enters the atmosphere from outer space. If its original temperature is 38°C, the effective average temperature of the atmosphere is 1093°C, and the effective heat transfer coefficient is , estimate the temperature of the shell after reentry, assuming the time of reentry is 10 min and the interior of the shell is evacuated.
3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.
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 1.18