7.36 A long, cylindrical, electrical heating element of diameter D = 12 mm, thermal conductivity k = 240 W/m. K, den- sity p = 2700 kg/m³, and specific heat c, = 900 J/kg . K is installed in a duct for which air moves in cross flow over the heater at a temperature and velocity of 30°C and 8 m/s, respectively. (a) Neglecting radiation, estimate the steady-state sur- face temperature when, per unit length of the heater, electrical energy is being dissipated at a rate of 1000 W/m. (b) If the heater is activated from an initial temperature of 30°C, estimate the time required for the surface temperature to come within 10°C of its steady-state value.

The thermal boundary layer in heat transfer exists whenever there is a difference between the temperatures of the fluid and the outside environment. For fluid flow, how does a boundary layer form?A. Because common surfaces are not smooth, the flow is hindered, and boundary layers are formed.B. The disturbances on the streamlines in high Reynolds number flow produces boundary layer.C. The existence of walls or “boundaries” that keep the fluid inside (or outside) creates the boundary layer.D. Velocity profiles are inherently approaching no-slip condition because of the resistance of the walls.

Q1/ Consider the following fluids, each with a velocity of u = 9 m/s and a temperature of 10°C, in cross flow over a 10-mm diameter cylinder maintained at 50°C. Calculate the rate of heat transfer per unit length for saturated water? ***** 2 Satu Dety PP Liquid 3149 9920 4.246 996.0 528 9040 7.354 902.1 393 9901 Nu 0.3+1 Endalay Specific Heal Veputation 0.62 Re2 Pro [1 + (0.4/Pypa[1+ (282,000) aper 0.0231 2402 4180 00304 4378 0.0397 2419 4178 0.0612 2407 4179 00655 2335 4180 tree Thermal Coductivity THAY Liquid Vapor Le Dynamic Vacosity 1870 Vapor Liquid 0607 0.0186 0110 0.90710 1875 0615 0.0169 0.796 101 100110 1880 BAZ3 0.0192 0720x10 1016 10 1825 0.631 0.5196 0653x10 103150 1832 0437 0.0000 0596x10104610 Fran www. Lignon A'IN Ligad Vaper 614 1.00 0.24710 542 100 0234-10 4.83 1.00 0.337x10 4.32 100 0377x10 191 100 0415-10

Typically, air is heated in a hair dryer by blowing it across a coiled wire through which an electric current is passed. Thermal energy is generated by electric resistance heating within the wire and is transferred by convection from the surface of the wire to the air. Consider conditions for which the wire is initially at room temperature, T;, and resistance heating is concurrently initiated with airflow att = 0. q Air Too, h 001 Coiled wire (ro, L, k, p, cp) 0000 Pelec Airflow ווי SU