An incandescent lightbulb is an inexpensive but highly inefficient device that converts electrical energy into light. It converts about 10 percent of the electrical energy it consumes into light while converting the remaining 90 percent into heat. The glass bulb of the lamp heats up very quickly as a result of absorbing all that heat and dissipating it to the surroundings by convection and radiation.
Consider an 8-cm-diameter 60-W lightbulb in a room at 258C. The emissivity of the glass is 0.9. Assuming that 10 percent of the energy passes through the glass bulb as light with negligible absorption and the rest of the energy is absorbed and dissipated by the bulb itself by natural convection and radiation, determine the equilibrium temperature of the glass bulb. Assume the interior surfaces of the room to be at room temperature.

The surface temperature of the bulb is 169.3°C

Explanation:

The heat generated by the electric bulb is transferred by convection and radiation into the surrounding air. In-order to find the surface temperature of the bulb, first evaluate the properties of the surrounding medium and then find the Rayleigh number. From the Rayleigh number, estimate the heat transfer coefficient with the help of Nusselt number. The heat transfer coefficient when substitute in the heat generated equation, gives the final temperature.

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