A simple procedure for measuring surface convection heat transfer coefficients involves coating the surface with a thin layer of material having a precise melting point temperature. The surface is then heated and, by determining the time required for melting to occur, theconvection coefficient is determined. following experimental arrangement uses the procedure to determine the convection coefficient for gas flow normal to a surface. Specifically, a long copper rod is encased in a super insulator of very low thermal conductivity, and a very thin coating is applied to its exposed surface.
If the rod is initially at
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Fundamentals of Heat and Mass Transfer
- 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 1.4arrow_forwardA section of a composite wall with the dimensions shown below has uniform temperatures of 200C and 50C over the left and right surfaces, respectively. If the thermal conductivities of the wall materials are: kA=70W/mK,kB=60W/mK, kC=40W/mK, and kP=20W/mK, determine the rate of heat transfer through this section of the wall and the temperatures at the interfaces. Repeat Problem 1.34, including a contact resistance of 0.1 K/W at each of the interfaces.arrow_forwardBoth ends of a 0.6-cm copper U-shaped rod are rigidly affixed to a vertical wall as shown in the accompanying sketch. The temperature of the wall is maintained at 93C. The developed length of the rod is 0.6 m, and it is exposed to air at 38C. The combined radiation and convection heat transfer coefficient for this system is 34W/m2K. (a) Calculate the temperature of the midpoint of the rod. (b) What will the rate of heat transfer from the rod be?arrow_forward
- I need the answer as soon as possiblearrow_forwardStainless steel (AISI 304) ball bearings, which have been uniformly heated to 850°C, are hardened by quenching them in an oil bath that is maintained at 40°C. The ball diameter is 12.5 mm, and the convection coefficient associated with the oil bath is 1000 W/m².K. (a) If quenching is to occur until the surface temperature of the balls reaches 100°C, how long, in s, must the balls be kept in the oil? What is the center temperature, in °C, at the conclusion of the cooling period? (b) If 10,000 balls are to be quenched per hour, what is the rate at which energy must be removed, in kW, by the oil bath cooling system in order to maintain its temperature at 40°C? Evaluate the properties of the stainless steel (AISI 304) at 500 K. Part A If quenching is to occur until the surface temperature of the balls reaches 100°C, how long, in s, must the balls be kept in the oil? What is the center temperature, in °C, at the conclusion of the cooling period? t = T₁ = i i S °℃arrow_forwardThermal treatment of a steel cube, of 10 cm side length, L, requires a two step cooling process. In the first step, the cube at the uniform temperature of 500°C is placed in air where it is cooled to 250°C. In the second step, it is moved to large oil bath of temperature 20°C where the heat transfer coefficient at all surfaces is the same and equal to h = 50 W m-2 K-1. %3D Calculate the total amount of energy lost by the cube and show that the lumped capacitance method can be used to calculate the temperature during the second step of cooling. Calculate the time necessary to reduce the cube temperature to 50°C. Thermal properties Density = 8000 kg m-3 Cp= 480 J kg-1 K-1 k = 30 W m-1 K-1. %3Darrow_forward
- Sepleniber 02 Heat is generated in a homogeneous spherical piece of radioactive material (2D - 8 cm) at rate of 71.8 MW/m and is dissipated to the environment. The center and surface temperatures of the spheres are (491°C and 80°C) respectively, What is the thermal conductivity of the radioactive material "W/m.K"?arrow_forwardAn underwater sonar that maps the ocean bathymetry is encapsulated in a sphere with a diameter of 85 mm. During operation, the sonar generates heat at a rate of 300W. What is the sonar surface temperature when it’s located in a water column where the temperature is 15o C and the water current is 1 m/sec? The sonar was pulled out of the water without turning it off, thus, it was still working. The air temperature was 15o C and the air speed was 3 m/sec. What was the sonar surface temperature? Was there any reason for concern?arrow_forwardDon’t use Heissler charts to answer this question Heat sterilization of lumber, timbers, and pallets is used to kill insects to prevent their transfer between countries in international trade. This is analogous to food sterilization by heat. A typical requirement here is that the slowest heating point of any woodconfiguration be held at 56 °C for 30 minutes. Consider hot air heating of wooden boards that maintains their surface temperature at 70 °C. The boards are stacked outside and in the winter time they can be considered to be at 0 °Cwhen theyare brought in for heating. The thermal diffusivity of the wood is 9*10-8m2/s. a.Calculate the time from the start of heating for a 2.5 cm thick board to reach a sterilization temperature of 56 °C at its slowest heating point .b.Calculate the heating time when four such boards are stacked together. c.Calculate the ratio of the two heating times (for a single board versus when they are stacked), and explain the ratio. Note: You’re free to…arrow_forward
- To reduce heat flux, the skin is wrapped with a clothing material. What should be the thickness of the clothing material covering the surface of this skin tissue to reduce the heat flux to half of its original value? What is the temperature at the skin-clothing material interface? Note: if you think you need to have more information to solve this problem, you're free to make assumptions. Please state them clearly in your answer, if you need to make such assumptions.arrow_forwardThe wall of an industrial furnace, a 0.15 m thick chamotte with a heat conduction coefficient of 1.7 M / m.K It is made of bricks. Measurements made during continuous operation, internal and external surface temperatures showed that it was 1400 and 1150 K respectively. With side lengths of 0.5 m and 3 m calculate the heat passing through the wall per unit timearrow_forward(B) In order to measure the convection heat transfer coefficients one of the common methods involve bonding one surface of a thin metallic foil to an insulating material and exposing the other surface to the fluid flow conditions of interest. T, h -Foil (Pälec T) - Foam Insulation (k) Figure1: Heat transfer direction By passing an electric current through the foil, heat is dissipated uniformly within the foil and the corresponding flux (P elec) may be inferred from related voltage and current measurements. Consider conditions for which To = Tb = 25°C, Pelec = 2000 W/m2, L = 7 mm, and k = 0.05 W/m.K. (1) With water flow over the surface, the foil temperature measurement yields Ts= 30°C. Determine the convection heat transfer coefficient for both cases below. The foil has an emissivity of 0.15 and is exposed to large surroundings at 25°C. (2) On the other hand, if air flows over the surface and the temperature measurement yields Ts = 120°C, how the convection heat transfer coefficient…arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning