Question

Consider the electrical heater of Problem 7.49. If the blower were to malfunction, terminating airflow while...

Consider the electrical heater of Problem 7.49. If the blower were to malfunction, terminating airflow while the heater continued to operate at 1000 W/m, what tem- perature would the heater assume? How long would it take to come within 10 C of this temperature? Allow for radiation exchange between the heater ( 0.8) and the duct walls, which are also at 27 C.

7.49) Along,cylindrical,electricalheatingelementofdiameter D 10 mm, thermal conductivity k 240 W/m K, den- sity 2700 kg/m3, and specific heat cp 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 27 C and 10 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 27 C, estimate the time required for the surface temperature to come within 10 C of its steady-state value.

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