I am checking the cooling effect of water and silicone oil in a heat sink. The flow rate which I am using is 0.1 ml/min to 100 ml/min. In usual case, when we measure the the thermal resistance of the heat sink we calculate the conduction resistance for the base plate of the heat sink, and convection and capacity resistance for the liquid. I wanted to ask that how can we measure the conduction resistance for the liquid because one of the reason why water has better cooling is because it has high thermal conductivity then silicone oil but when we do the resistance analysis of the heat sink the thermal conductivity for liquids is not used.
Hi Muhammed
You can calculate the flow resistance by using the below equation:
R_flow=1/(mass_dot*cp_air)
Flow resistance=1/ mass flow rate of liquid per liquid specific heat
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Then, to measure the conductance resistance of liquid by:
Transport phenomena and metals properties
A.K. Lahiri, in Fundamentals of Metallurgy, 2005
Thermal conductivity
According to the kinetic theory of gases, the thermal conductivity of monatomic gases is independent of pressure and is proportional to the square root of temperature. The predicted pressure dependence is valid up to 10 atmospheres but temperature dependence is too weak. The temperature dependence of the thermal conductivity of gas can be expressed as
k=k0(T/T0)^n ………………(1)
where k0 is thermal conductivity at T0 K. Eucken’s equation
k=(Cp+1.25R/M)μ ………….(2)
is widely used for the estimation of the thermal conductivity of gases. Cp, R, M and μ are respectively specific heat, gas constant, molecular weight and viscosity. Table 5.6 gives the thermal conductivity of some common gases. It shows that the thermal conductivity of hydrogen is much higher than other gases.
Thamer Salem Thank you for responding to my query. The thermal conductivity equations mentioned are valid for gases, so can we use them for liquid. Also, I am using water as coolant and we know its thermal conductivity. I want to know that how can we measure the conduction resistance for the liquid in the heat sink problem. When we say that water is a good coolant compared to other liquids e.g. silicone oil, one of the reason is the high conductivity of the water. However equations used for the thermal resistance analysis of the heat sink do not contain thermal conductivity term. I wanted to know how we can quantify the thermal conductivity effect.
If you want a simple, reasonably accurate technique for measuring thermal conductivity of a liquid, research 'Transient Hot Wire' methods. With common lab equipment and some care you can get good results without too much difficulty. Certainly results that are within the uncertainty of typical convective heat transfer problems.
Paul Borthwick Thank you for responding to my query. I do not want to measure the thermal conductivity. I want to know that how can we calculate conduction resistance of a liquid in heat sink cooling problem.
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