What is the fin effectiveness of a rectangular plate fin made of aluminum O.3 mm thick mounted on a 16-mm-OD tube if the vertical tube spacing is 50 mm and the horizontal spacing is 40 mm? The air-side heat-transfer coefficient is 65 W/m2 • K, and the conductivity of aluminum is 202 W/m • K.
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What is the fin effectiveness of a rectangular plate fin made of aluminum O.3 mm thick mounted on a 16-mm-OD tube if the vertical tube spacing is 50 mm and the horizontal spacing is 40 mm? The air-side heat-transfer coefficient is 65 W/m2 • K, and the conductivity of aluminum is 202 W/m • K.
Ans. ƞ is 0. 72
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- 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 1.41.10 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.One end of a 0.3-m-long steel rod is connected to a wall at 204C. The other end is connected to a wall that is maintained at 93C. Air is blown across the rod so that a heat transfer coefficient of 17W/m2 K is maintained over the entire surface. If the diameter of the rod is 5 cm and the temperature of the air is 38C, what is the net rate of heat loss to the air?
- 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.*** **answer is correct To increase the rate of heat transfer from a device, which has 0.1 m² of exposed surface, you added 500 pin fins of 4 cm length and 2 mm diameter on top of the device's exposed surface. How much (i.e., by what factor) did the rate of total heat transfer rate (from the surface) increase? Assume that the convection coefficient is 42 W/m².K, and the thermal conductivity of the fin material is 240 W/m.K. Neglect radiation. 2:06nt Select D 0 261.40 ( 142. СС 60 Home Insert Space 300c° Insert Draw View Exercise 5 the bobul 3-75 Consider a 5-m-high 8-m-long, and 0.22-m-thick wall whose representative cross section is as given in the figure. The thermal conductivities of various materials used, in W/m-K, are k, = kp = 2, kg = 8, ke=20, kp = 15, and kg = 35. The left and right surfaces of the wall are maintained at uniform temperatures of 300°C and 100°C, respectively. Assuming heat transfer through the wall to be one-dimensional, determine (a) the rate of heat transfer through the wall; (b) the temperature at the point where the sections B, D, and E meet; and (c) the temperature drop across the section F. Disregard any contact resistances at the interfaces. RA p Re RB RC Ro RE ******** ве R=RA + ROBC+R DE + Rr ****** 300°C Ri 1000 1 сгу اللينفب اللوطن يمني نفتي المتريال 5 cm | +100 cm оста 6 cm Pencil + 100°C 0.12 M 10 C
- Use the VG-1 Performance Evaluation Criteria (PEC) (assume all thermal resistance on one side) to compare the performance of the offset strip-fin (OSF) and the plain surface geometry (use subscript “s”). Assume that the plain fin geometry operates at Res = 834, at which js = 0.00669 and fs = 0.0235. Assume that curve fits of f and j vs. Re exist for the OSF geometry; let these be f = F(Re) and j = J(Re). Derive the equation to calculate the Re at which the OSF geometry would operate for the VG-1 case. Your final expression should include all of the above numerical values and expressions for f and j. Describe how you would solve for Re. Assume that both surfaces have the same hydraulic diameter.2. Calculate insulation thickness (minimum value) required for a pipe carrying steam at 180°C. The pipe size is 8" and the maximum allowable temperature of outer wall of insulation is 50 °C. Thermal conductivity of the insulation material for the temperature range of the pipe can be taken as 0.04 W/m K. The heat loss from steam per meter of pipe length has to be limited to 80 W/m. ANSWER: 102.5 mmA dormitory at a large university, built 50 years ago, has exterior walls constructed of Ls a thermal conductivity of k, = 25-mm-thick sheathing with = 0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the = 0.029 W/m:K to entire dormitory by applying an L; = 25-mm-thick layer of extruded insulation characterized by k; the exterior of the original sheathing. The extruded insulation is, in turn, covered with an Lg = 5-mm-thick architectural glass with kg exterior air temperatures are T; 1.4 W/m-K. Determine the heat flux through the original and retrofitted walls when the interior and = 22°C and T. 0°C, respectively. The inner and outer convection heat transfer 0,i coefficients are h; = 5 W/m2.K and ho 25 W/m2-K, respectively. The heat flux through the original walls is i W/m?. The heat flux through the retrofitted walls is i W/m?.
- Steel pipe (outer diameter 100 mm) is covered with two layers of insulation. The inner layer, 40 mm thick, has a thermal conductivity of 0.07 W / (m K). The outer layer, 20 mm thick, has a thermal conductivity of 0.15 W / (m K). Pipes are used to delivering steam with a pressure of 600 kPa. The temperature on the outer insulation surface is 24 ° C. If the pipe is 10 m long, determine the following: (assuming that the conduction heat transfer resistance of the steel pipe and the vapor convection resistance are negligible). a. Hourly heat loss ... (kj / hr)b. temperature between insulation layers ... (° C.)Steel pipe (outer diameter 100 mm) is covered with two layers of insulation. The inner layer, 40 mm thick, has a thermal conductivity of 0.07 W / (m K). The outer layer, 20 mm thick, has a thermal conductivity of 0.15 W / (m K). Pipes are used to deliver steam with a pressure of 800 kPa. The temperature on the outer insulation surface is 24 ° C. If the pipe is 10 m long, determine the following: (assuming that the conduction heat transfer resistance of the steel pipe and the vapor convection resistance are negligible). a. Heat loss per hour. = AnswerkJ / hr. b. Temperature between insulation layers. = Answer ° C.Steel pipe (outer diameter 100 mm) is covered with two layers of insulation. The inner layer, 40 mm thick, has a thermal conductivity of 0.07 W / (m K). The outer layer, 20 mm thick, has a thermal conductivity of 0.15 W / (m K). Pipes are used to deliver steam with a pressure of 800 kPa. The temperature on the outer insulation surface is 24 ° C. If the pipe is 10 m long, determine the following: (assuming that the conduction heat transfer resistance of the steel pipe and the vapor convection resistance are negligible). a. Heat loss per hour. = Answer kJ / hour. b. Temperature between insulation layers. = Answer ° C.