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An array of 10 silicon chips, each of length
If the temperature of each chip may not exceed 80°C, what is the maximum allowable power per chip? What is the maximum allowable power if a turbulence promoter is used to trip the boundary layer at the leading edge? Would it be preferable to orient the array normal, instead of parallel, to the airflow?
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Fundamentals of Heat and Mass Transfer
- O A vertical cylinder 6 ft tall and 1 ft in diameter might be used to approximate a man for heat-transfer purposes. Suppose the surface temperature of the cylinder is 78°F, h=2 Btu/h - ft2 .°F, the surface emissivity is 0.9, and the cylinder is placed in a large room where the air temperature is 68°F and the wall temperature is 45°F. Calculate the heat lost from the cylinder. Repeat for a wall temperature of 80°F. What do you conclude from these calculations?arrow_forward7 An electric heater is made of thin conducting wires forming a mesh. The diameter of the wire is 0.5 mm and the total length of all the wires is 2 m. The temperature of the wire is 1,000 K. The heat from the wire is dissipated to the air at 300 K flowing through the wire mesh at a velocity of 5 m/s. Ignore radiation of the heater and assume it has a uniform temperature. T Const = (a) Calculate the convection heat transfer coefficient(h) between the air and the heater. (b) If the total electrical resistance of the heater is 10 2, what is the electrical current? (c) Calculate the Biot number of the heater. V=IR (d) The electrical current is suddenly cut off. How long does it take for the heater temperature to drop to 500 K? Properties of the air: pa = 0.7 kg/m³, Cp,a= 1030 J/kg K, va = 38.8×106 m²/s, ka=0.041 W/m•K, aa =56.7×106 m²/s, and Pra = 0.684. Properties of the heater: ph = 8900 kg/m³, Cp,h= 380 J/kg K, and kn = 400 W/m.K.arrow_forwardYou are designing an enclosure which houses a PCB. Mounted outside the enclosure on its top is a set of aluminum fins, as seen in the sketch below. The enclosure (and fins) are 150mm deep long. There is no forced airflow. Neglect the thickness of the enclosure walls. Neglect heat transfer out of the sides and bottom of the enclosure. If the PCB produces 4000 W/m2: a) Most industrial-grade ICs have a maximum temperature of 85oC. If you must keep your PCB below this what is the maximum thermal resistance from the PCB to the air surrounding the fins? Assume the air around the fins stays at room temperature. b) If the top of the enclosure (and bottom of the fin base) runs at 50°?, what is the effective thermal conductivity above the PCB? Hint: Because the air inside the enclosure is trapped and heats up, any air properties NOT given below are calculated at the average air temperature (T1+T2)/2. c) What is the temperature at the base of the fins? d) What is the efficiency of the fins? e)…arrow_forward
- A bare fuel rod has a diameter of 0.373 in and length of 12 ft. The volumetric heat generation rate for this rod is q ′′′ = 10,000 kW/ft3 . The heat produced by the rod is removed by water used as the coolant, at 2250 psia and 575 F. Find the heat transfer coefficient between the bare fuel rod and the coolant. The fuel rod surface temperature is 675 F. [Ans.: h = 2651 Btu/h·ft2 ·F].arrow_forwardLet's say a 3.0 gram copper wafer is dropped from a height of 50.0 meters. If 60% of the potential energy lost in the drop could be converted to thermal energy used to heat the copper from an initial temperature of 25 degrees celsius, what would the final temperature of the copper wafer? Would the answer be different if the wafer has a mass greater than 3 grams? Note: the specific heat of copper is 387 J/(kg*K). The temperature is between 25.8 and 26.0 degrees celsius, yes the bigger the mass the greater the energy. O The temperature is between 25.6 and 25.8 celsius, answer does not depend on mass. O The temperature is between 25.0 and 25.2 celsius, answer does not depend on mass. O The temperature is 25.5 and of course the more mass something has the greater energy will be needed to raise the temperature. The temperature is 26.2 and if the mass is doubled so will be the change in temperature. O The temperature is 25.9 degrees celsius and the answer does not depend on mass. O The…arrow_forwardFast plz hand written.... I'll upvote if hand written plz asaparrow_forward
- If hand written i'll upvote otherwise downvote?...arrow_forwardJohn is employed by a new food processing plant where eggs are to be boiled in large quantities to produce egg salad. John has been assigned the task of determining the minimum cooking time for the eggs, utilizing his vast knowledge of heat transfer. As a basis of calculation, he assumed that the eggs are perfect spheres with a diameter of 5 cm and uniform material properties (r =1000kg/m'; Cp = 3300 J/(kg K) and k=0.75 W/(m-K)). The eggs were taken from a cold storage room at 5°C and immediately put into pots of boiling water at 100°C. The convective heat transfer coefficient of the water is h = 1500 W/m°K. For health safety reasons, the plant requires that the entire egg must be at a temperature above 75°C for at least 20 seconds in order for the cooking process to be complete. a. What is the minimum time necessary to cook the egg to company specifications? b. In the testing lab, John forgot to remove a batch of eggs, and they remained in the cooking pot long enough to reach a…arrow_forwardA vertical cylinder 6 ft tall and 1 ft in diameter might be used to approximate a man for heat-transfer purposes. Suppose the surface temperature of the cylinder is 78°F, h=2 Btu/h · ft2 . °F, the surface emissivity is 0.9, and the cylinder is placed in a large room where the air temperature is 68°F and the wall temperature is 45°F. Calculate the heat lost from the cylinder. Repeat for a wall temperature of 80°F. What do you conclude from these calculations?arrow_forward
- HEAT TRANSFER A 0.5 cm diameter copper wire is exposed to environment air in cross flow over the wire with uniform velocity of 0.5 m/s. The copper wire is insulated by a rubberized sheath with thermal conductivity of 0.13 W/m∙K. The outer surface of the wire has a temperature of 60˚C due to the electric current flowing through it. The thermo-physical properties of the environment are υ =15.89 × 10-6 m2 /s, Pr = 0.707, k = 26.3 x 10-3 W/m.K. The outer surface of the insulation must be kept at temperature of 45˚C or below. Design a suitable thickness of the insulation in order toallow the use of such insulated wire in a room temperature of 27˚C.arrow_forwardANS : Q=114.93W/marrow_forward2, The internal diameter of a boiler generating tute is 4in. and the wall thickness is 0.375 in. In operation the external surrace film temperature is 26 Btu/hr . ft².°F, the internal film coefficient is 1700 Btu/hr.ft².°F and a 0.375 in. thickness scale having a conductivity of 0.6 Btu/hr.ft. °F is deposited on the inner surface of the tube. The metal conductivity is 26 Btu/hr.ft. °F, the furnace gas temperature is 2100 °F and the temperature of the water is 500 °F, calculate (a) the overall coefficient of heat transfer for the clean tube, (b) the overall heat transfer coefficient including the effect of the scale, (c) the external tube surface temperature before and after the scalearrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning