heat transfer question help

A hairdryer incorporates a 750 W electrical resistance heater. If the heater consists of 0.8 mm diameter wire and has a maximum temperature of 400°C, find the length of wire required if the air is heated from 20 to 60°C and the heat transfer coefficient for air flowing over the wire is 30 W / m^2 K

do we use q=HA delta T??

i cant think of an equating involving length
ans is 27.63m

What about the surface area of the wire?

Reply 2

The_Blade

OP

6

Original post by Smack

What about the surface area of the wire?

2 pi r L + 2 pi r^2

is the change in temp 40 degrees?

i rearranged to get L but wrong ans :frown:

Original post by The_Blade

2 pi r L + 2 pi r^2

is the change in temp 40 degrees?

i rearranged to get L but wrong ans :frown:

The temperature difference is the difference between the temperature in the wire and the bulk temperature of the fluid.

For the surface area of the wire, ignore the ends, because they don't exist, as it's a closed circuit, and even if they did, they'd likely be negligible relative to the area along the length of the wire anyway.

I got the correct answer using the above method.

Reply 4

The_Blade

OP

6

Original post by Smack

The temperature difference is the difference between the temperature in the wire and the bulk temperature of the fluid.

For the surface area of the wire, ignore the ends, because they don't exist, as it's a closed circuit, and even if they did, they'd likely be negligible relative to the area along the length of the wire anyway.

I got the correct answer using the above method.

so 750= 30 x (2 x pi x r x L) x 40?

what is bulk temp of fluid mean?

edit: got ans but question still applies ^^

(edited 9 years ago)

Reply 5

a10

19

Original post by The_Blade

what is bulk temp of fluid mean?

edit: got ans but question still applies ^^

The definition of convection is the bulk movement of a fluid i.e. its not just one small piece of air if you get what i mean lol

its always in "bulk" i.e. a huge proportion of it

Idk maybe smack can give you a better description lol