how to calculate thermal conductivity ?
2018-12-06Auther: Thermo Electronic Technology Co., Ltd
K is the thermal conductivity of the material itself, the inherent performance parameters, describe the strength of thermally conductive material. This feature with the material itself, the size, shape, and thickness are not related, but with the composition of the material itself has a relationship, so thermally similar material rates are the same, and will not change because the thickness is not the same.
Will merge the following two formulas can be obtained by K = d / R. Since the K value is constant, able to see the value of the thermal resistance R, the same material thickness d is directly proportional. Is to say the thicker the material, the thermal the greater the resistance.
Q = KAT / d,
R = AT / Q Q: heat, W
K: thermal conductivity, W / mk
A: contact area
d: heat transfer distance
T: temperature difference
R: thermal resistance
But if you look closely some material thermal conductivity material, you will find a lot of heat conducting material thermal resistance R, with the thickness d is not completely proportional relationship. This is because most are not thermally conductive material to form a single component, there is a corresponding non-linear changes thickness increases, the thermal resistance will increase, but not necessarily completely proportional linear relationship, a girl is steeper curve relationship.
According to R = AT / Q This formula, in theory be able to test and calculate the thermal resistance of a material R. However, this formula is only a basic formula for idealized conditions he set are: the contact surface is completely smooth and flat, all the heat by thermal conduction all the way through the material, and reaches the other end.
Reality it is not a girl's condition so tested and calculated thermal resistance of the material itself is not complete thermal resistance of the material itself should be called thermal resistance + contact surface thermal resistance. Because the contact surface flatness , smooth or rough, and the pressure to install fastening of different sizes, it will create a different contact surface thermal resistance, will arrive at different total thermal resistance.
So be recognized internationally popular set a standard test methods and conditions, even in the data will often see the ASTM D5470. This test method described resistance test was held, when the choice of how much of the contact area A, how much heat the value of Q, as well as the pressure applied to the value of the contact surface. We all apply the same method to test different materials, and obtained results, compared to only meaning.