An electrical wire with a length of 5-m and 3.5-mm-Ø spreads over one room at 18°C. Heat is generated in the wire due to resistance heating with a convection heat transfer coefficient of 23 W/m2-K between the outer surface of the wire and the air in the room. The voltage drop and electric current through the wire are measured to be 60 V and 2 A, respectively. Disregarding any heat transfer by radiation, determine the surface temperature of the wire (°C). Surface Temp. of Wire (°C)

An electrical wire with a length of 5-m and 3.5-mm-Ø spreads over one room at 18°C. Heat is generated in the wire due to resistance heating with a convection heat transfer coefficient of 23 W/m2-K between the outer surface of the wire and the air in the room. The voltage drop and electric current through the wire are measured to be 60 V and 2 A, respectively. Disregarding any heat transfer by radiation, determine the surface temperature of the wire (°C). Surface Temp. of Wire (°C)

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.19P: 1.19 A cryogenic fluid is stored in a 0.3-m-diameter spherical container is still air. If the...

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Heat is transferred at a rate of 0.1 kW through glass wool insulation (density=100kg/m3) with a 5-cm thickness and 2-m2 area. If the hot surface is at 70C, determine the temperature of the cooler surface.
1.19 A cryogenic fluid is stored in a 0.3-m-diameter spherical container is still air. If the convection heat transfer coefficient between the outer surface of the container and the air is 6.8 , the temperature of the air is 27°C, and the temperature of the surface of the sphere is –183°C, determine the rate of heat transfer by convection.
3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.
1.39 On a cold winter day, the outside wall of a home is exposed to an air temperature of when the inside temperature of the room is at . As a result of this temperature gradient, there is heat loss through the wall to the outside. Consider the convective heat transfer coefficients for the air inside the room and at the outside wall surface to be, respectively, 12.0 and . If the composite room wall is modeled as a plane wall with a thermal resistance per unit area of , determine the temperature at the outer surface of the wall as well as the rate of heat flow through the wall per unit area. If the homeowner were to consider using a fiberglass insulation layer on the inside wall surface for reducing this heat loss by 50%, what is the required thickness of this layer and the outside wall temperature for this case?
A cooling system is to be designed for a food storage warehouse for keeping perishable foods cool prior to transportation to grocery stores. The warehouse has an effective surface area of 1860 m2 exposed to an ambient air temperature of 32C. The warehouse wall insulation (k=0.17W/(mK)) is 7.5 cm thick. Determine the rate at which heat must be removed (W) from the warehouse to maintain the food at 4C.
1.60 Two electric resistance heaters with a 20 cm length and a 2 cm diameter are inserted into a well-insulated 40-L tank of water that is initially at 300 K. If each heater dissipates 500 W, what is the time required for bringing the water temperature in the tank to 340 K? State your assumption for your analysis.
Show that the rate of heat conduction per unit length through a long, hollow cylinder of inner radius ri and outer radius ro, made of a material whose thermal conductivity varies linearly with temperature, is given by qkL=TiTo(rori)/kmA where Ti = temperature at the inner surface To = temperature at the outer surface A=2(rori)/ln(ro/ri)km=ko[1+k(Ti+To)/2]L=lenthofcyclinder
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.
A person puts a few apples into the freezer at 15°C cool them quickly for guestswho are about to arrive. Initially, the apples are at a uniform temperature of 20°C,and the heat transfer coefficient on the surfaces is 8 W/m2·K. Treating the apples as9-cm-diameter spheres and taking their properties to be 840 kg/m3, Cp 3.81 kJ/kg·K, k = 0.418 W/m·K, and α =10-7 m2/s, determine the center and surface temperatures of the apples in 1 h. Also, determine the amount of heat transfer from each apple. Solve this problem using analytical one-term approximation method (notthe Heisler charts). Answer: Center: 11.2 ℃, Surface: 2.7 ℃, heat transfer: 17.2 kJ
Consider a 2-m-high electric hot-water heater thathas a diameter of 40 cm and maintains the hot water at 55°C.The tank is located in a small room whose average temperatureis 27°C, and the heat transfer coefficients on the innerand outer surfaces of the heater are 50 and 12 W/m2·K,respectively. The tank is placed in another 46-cm-diametersheet metal tank of negligible thickness, and the spacebetween the two tanks is filled with foam insulation (k =0.03 W/m·K). The thermal resistances of the water tank andthe outer thin sheet metal shell are very small and can be neglected. The price of electricity is $0.08/kWh, and the home owner pays $280 a year for water heating. Determine the fraction of the hot-water energy cost of this household that is due to the heat loss from the tank. Hot-water tank insulation kits consisting of 3-cm-thick fiberglass insulation (k = 0.035 W/m·K) large enough to wrap the entire tank are available in the market for about $30. If such an insulation is installed on…
Liquid flows in a metal pipe with an inner diameter of D1 = 20 mm and an outer diameter of D2 = 30 mm. The thermal conductivity of the pipe wall is 10 W/m⋅K. The inner surface of the pipe is coated with a thin polyvinylidene chloride (PVDC) lining. Along a length of 95 cm, the pipe outer surface is exposed to convection heat transfer with hot gas, at T∞ = 95° C and h = 6 W/m2 ⋅K, and thermal radiation with a surrounding at Tsurr = 95° C. The emissivity at the pipe outer surface is 0.3. The liquid flowing inside the pipe has a convection heat transfer coefficient of 52 W/m2⋅K. If the outer surface of the pipe is at 86° C, determine the temperature at the PVDC lining and the temperature of the liquid.The ASME Code for Process Piping (ASME B31.3-2014, A323) recommends a maximum temperature for PVDC lining to be 81° C. Does the PVDC lining comply with the recommendation of the code?
Steam at 235°C is flowing inside a steel pipe(k = 61 W/m·K) whose inner and outer diameters are10 cm and 12 cm, respectively, in an environment at 20°C.The heat transfer coefficients inside and outside the pipeare 105 W/m2·K and 14 W/m2·K, respectively. Determine(a) the thickness of the insulation (k = 0.038 W/m·K) neededto reduce the heat loss by 95 percent and (b) the thickness ofthe insulation needed to reduce the exposed surface temperatureof insulated pipe to 40°C for safety reasons.