# Heat Transfer on a windshield

Im doing a project to put a heater in a car to pre heat it. I need to determine how many watts would be needed to defrost a window in a certain amount of time. I know the temp of the outside air and temperature to melt the ice is at least 0 C. I thought i could use it as a semi- infinite solid, but then how is the thickness of the glass taken into account. Also, does the velocity at which the air comes out of the vent matter and how does that fit into the equation. Any help would be appreciated.

## Answers and Replies

yup. The velocity of the air relative to the windshield (inside and outside) is accounted for in the convective coefficients. Essentially, I think you would have to set up a thermal circuit analysis to figure out how much heat it would take to melt the ice.

i second on drendank, and the glass can be accounted for as conduction in the thermal circuit.

The quantity of air being heated is more important than the velocity. The velocity can help you determine the quantity, if you know the size of the opening. Once you know the quantity, you can determine the temperature difference you need (rise in temperature from the entering temp to the leaving temp) to overcome the heatloss (if you don't overcome the heatloss, you will never get the temperature to rise).

Some questions to answer for your calculations:

Is the car outside or in a garage?

Is the car in sunshine or shade?

Ambient (to the car) conditions?

Size of the car's interior to be heated?

Amount of glass?

What temperature do you want to heat the car to?

How fast do you want to achieve that temperature?

wait a minute, isn't it a question of only melting the ice on the windshiled? it can't be inside heating and where he had taken it as a semi-infinite solid !

My mistake. He does mention "defrost." Is he going to spot heat the windshield only?

at first i thought like u did, that he wanted to heat the whole car, but semi-infinite solid really puts the dots on the letters, doesn't it !

at first i thought like u did, that he wanted to heat the whole car, but semi-infinite solid really puts the dots on the letters, doesn't it !
Yes it does. Raises some questions in my mind about the method to be used to distribute the heat.

Yeah, Im going to run an electric heater and i need to know how much power it will need just to heat the windshield enough to melt the ice in a certain amount of time.

The heater will be ran through the stock heating vents.

in this case its a simple, conduction + Convection problem. if u have a respectable Heat transfer book u can get an equation that defines the heat transfer from an inclined plane to the atmosphere, and conduction is simple. and u don't have to take a semi-infinite solid, because the equations have an area factor, whether it is the convection equation of the conduction, so u can get an exact value instead of a near to value.