John H. Lienhard IV, John H. Lienhard V. A Heat Transfer Textbook (776116), страница 65
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The receiver stores extra refrigerant for the system andhelps to regulate the pressure. The receiver is at equilibriumwith surroundings at 330 K. A 5 mm diameter, spherical thermistor inside the receiver monitors the liquid level. The thermistor is a temperature-sensing resistor driven by a small electric current; it dissipates a power of 0.1 W.
When the systemis fully charged with refrigerant, the thermistor sits below theliquid surface. When refrigerant leaks from the system, the liquid level drops and the thermistor eventually sits in vapor. Thethermistor is small compared to the receiver, and its power istoo low to affect the bulk temperature in the receiver.a. If the system is fully charged, determine the temperatureof the thermistor.b. If enough refrigerant has leaked that the thermistor sits invapor, find the thermistor’s temperature.
Neglect thermalradiation.8.52Ammonia vapor at 300 K and 1.062 MPa pressure condensesonto the outside of a horizontal tube. The tube has an O.D. of1.91 cm.a. Suppose that the outside of the tube has a uniform temperature of 290 K. Determine the average condensation452Chapter 8: Natural convection in single-phase fluids and during film condensationheat transfer cofficient of the tube.b. The tube is cooled by cold water flowing through it andthe thin wall of the copper tube offers negligible thermalresistance. If the bulk temperature of the water is 275 Kat a location where the outside surface of the tube is at290 K, what is the heat transfer coefficient inside the tube?c.
Using the heat transfer coefficients you just found, estimate the largest wall thickness for which the thermal resistance of the tube could be neglected. Discuss the variation the tube wall temperature around the circumferenceand along the length of the tube.8.53An inclined plate in a piece of process equipment is tilted 30◦above the horizontal and is 20 cm long and 25 cm wide (in thehorizontal direction). The plate is held at 280 K by a stream ofliquid flowing past its bottom side; the liquid in turn is cooledby a refrigeration system capable of removing 12 watts fromit.
If the heat transfer from the plate to the stream exceeds 12watts, the temperature of both the liquid and the plate willbegin to rise. The upper surface of the plate is in contactwith gaseous ammonia vapor at 300 K and a varying pressure.An engineer suggests that any rise in the bulk temperature ofthe liquid will signal that the pressure has exceeded a level ofabout pcrit = 551 kPa.a.
Explain why the gas’s pressure will affect the heat transferto the coolant.b. Suppose that the pressure is 255.3 kPa. What is the heattransfer (in watts) from gas to the plate, if the plate temperature is Tw = 280 K? Will the coolant temperature rise?Data for ammonia are given in App. A.c.
Suppose that the pressure rises to 1062 kPa. What is theheat transfer to the plate if the plate is still at Tw = 280 K?Will the coolant temperature rise?References[8.1] W. Nusselt. Das grundgesetz des wärmeüberganges. Gesund. Ing.,38:872, 1915.References[8.2] C. J. Sanders and J. P. Holman. Franz Grashof and the GrashofNumber.
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