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Fundamentals of Heat and Mass Transfer
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- What is the approximate temperature difference between a hot plate and the surrounding air if the heat flux from the plate is 800 W/m2? Assume that the air is flowing past the surface with a velocity of 5 m/s giving a heat transfer coefficient of 20 W/(m2K).arrow_forwardHot water, k=48 W/m. It flows along the cast iron pipe, which is oC, at a speed of 1.4 m/s. The inner and outer diameter of the pipe are 3.0 and 3.5 cm, respectively. The pipe is located in a 15 m section of a basement floor with a temperature of 15 oC. If the water temperature drops from 70 OC to 67 OC when passing through the basement, and the film heat transfer coefficient on the inner surface of the pipe is 400 W/m2. If oC is, calculate the convective heat transfer coefficient on the outer surface of the pipe and the heat that will be transferred.arrow_forwardA computer cooled by a fan contains eight printed circuit boards (PCBs), each dissipating 10 W of power. The height of the PCBs is 12 cm and the length is 18 cm. The clearance between the tips of the components on the PCB and the back surface of the adjacent PCB is 0.3 cm. The cooling air is supplied by a 10-W fan mounted at the inlet. If the temperature rise of air as it flows through the case of the computer is not to exceed 10°C, determine (a) the flow rate of the air that the fan needs to deliver, (b) the fraction of the temperature rise of air that is due to the heat generated by the fan and its motor, and (c) the highest allowable inlet air temperature if the surface temperature of the components is not to exceed 70°C anywhere in the system. As a first approximation, assume flow is fully developed in the channel. Evaluate properties of air at a bulk mean temperature of 25°C. Is this a good assumption?arrow_forward
- An oil cooler consists of a system of nested identically centered pipes. The inner tube of this cooler has an inner diameter of 1.4 cm and a thickness of 0.2 cm. The outer surface of the cooler is insulated. The oil flows through the inner pipe at a speed of 4.0 kg/min, and the cooling water flows between the two pipes at a speed of 5.0 kg/Dec in the OPPOSITE direction to the oil flow. The hot oil is cooled from 400 OC to 180 OC. The cooling water inlet temperature is 18 OC. The film heat transfer coefficient between the surface of the pipe with oil 2100 W/m2 K heat transfer coefficient between the film and the surface with water 4070 W/m2 K is the pipe thickness of the heat transfer resistance is neglected. Calculate the length of the heat exchanger to be used. Cp water=1 cal/g oC; Cp,oil=1727 J/kg oCarrow_forwardHot water, k=48 W/m. It flows at a speed of 1.4 m/s along the cast iron pipe at oC. The inner and outer diameters of the pipe are 3.0 and 3.5 cm, respectively. The pipe is located within 15 m of a basement with a temperature of 15 °C. If the temperature of the water drops from 70 oC to 67 oC while passing through the basement and the film heat transfer coefficient on the inner surface of the pipe is 400 W/m2. Calculate the convective heat transfer coefficient on the outer surface of the pipe and the heat to be transferred.arrow_forwardHot air leaving the stove in the kitchen of a house at 70 0C, with a square section of 22 cm x 22 cm. sheet metal enters the 12 m long section of the channel with a speed of 6 m/s. Emissivity 0.3 The outer surface of the duct, whose thermal resistance is neglected, is located in the basement of the house at 14 0C and 12 W/m2 . It is in contact with cold air with 0C convection heat transfer coefficient. Basement Calculate the exit temperature of the hot air from the basement by taking the wall temperatures of the floor to 14 0C .arrow_forward
- Metal plates (k = 150 W/m·K, ρ= 2800 kg/m3, and cp = 900 J/kg·K) with a thickness of 2 cm exiting an oven are conveyed through a 10-m long cooling chamber at a speed of 4 cm/s, as shown in the figure. The plates enter the cooling chamber at an initial temperature of 500°C. The air temperature in the cooling chamber is 15°C, and the plates are cooled with blowing air and the convection heat transfer coefficient is given as a function of the air velocity h(V) = 33V0.8, where h is in W/m2·K and V is in m/s. To prevent any hazard to workers handling the plates, it is necessary to design the cooling process such that the plates exit the cooling chamber at a relatively safe temperature of 50°C or less. Determine the air velocity V and the heat transfer coefficient h such that the temperature of the plates exiting the cooling chamber is at 50C.arrow_forwardWater enters a 5-mm-diameter and 13-m-long tube at 15ºC with a velocity of 0.3 m/s, and leaves at 45ºC. The tube is subjected to a uniform heat flux of 2000 W/m2 on its surface. The temperature of the tube surface at the exit is(a) 48.7ºC (b) 49.4ºC (c) 51.1ºC (d) 53.7ºC (e) 55.2ºC(For water, use k = 0.615 W/m⋅°C, Pr = 5.42, ν =0.801×10-6 m2/s)arrow_forwardThe blades of a wind turbine turn a large shaft at a relatively slow speed. The rotational speed is increased by a gearbox that has an efficiency of 0.93. In turn, the gearbox output shaft drives an electric generator with an efficiency of 0.95. The cylindrical nacelle, which houses the gearbox, generator, and associated equipment, is of length L = 6 m and diameter D = 3 m. If the turbine produces P = 2.5 MW of electrical power, and the air and surroundings temperatures are T = 25 oC and Tsur = 20 oC, respectively, determine the minimum possible operating temperature inside the nacelle. The emissivity of the nacelle is 0.83, and the convective heat transfer coefficient is h = 35 W/m2 .K. The surface of the nacelle that is adjacent to the blade hub can be considered to be adiabatic, and solar irradiation may be neglected. Use Fin or N number of fins to reduce the Ts of the nacelle less than 143 oCarrow_forward
- Water (cp=4.182 kJ/kg.K, k=0.6405 W/m.K, Pr=3.57, ν=0.5537x10-6 m2/s, ρ=998 kg/m3) flows through a tube (D=0.5 cm) with an average velocity of 0.2 m/s. The average temperatures of the water at the inlet and outlet cross-sections are 16,21 oC and 78,49 oC, respectively. The uniform heat flux of 0,58 W/cm2 is subjected to the surface of the tube for heating process. Determine the tube length [m].arrow_forwardWater (cp=4.182 kJ/kg.K, k=0.6405 W/m.K, Pr=3.57, ν=0.5537x10-6 m2/s, ρ=998 kg/m3) flows through a tube (D=0.5 cm) with an average velocity of 0.2 m/s. The average temperatures of the water at the inlet and outlet cross-sections are 20 oC and 79,87 oC, respectively. The uniform heat flux of 0,69 W/cm2 is subjected to the surface of the tube for heating process. Determine the maximum surface temperature [oC] of the tube.arrow_forwardA plate with negligible thickness of 50 cm and a length of 100 cm is exposed to 2 different air flows in the same direction from its lower and upper surface along its length. The speed of the air flowing from the upper surface of the plate is 3600 m / min and its temperature is 473 K, the temperature of the air flowing from the lower surface is 298 K and its speed is 600 m / min. Calculate the total heat exchange between both fluids. (Assume the initial temperature of the plate is 160⁰C.)arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning