2.5 Microchannel cooling resu/ts
2.5.2 Water measurements and calculations
The cooling capacity of microchannel cooling with water as working fluid has been tested as a function of cooling flow. The power input is set at 105 Watt. Only one power setting is used sirree the thermal resistance is only slightly dependent on the power input, through temperature dependent properties such as conductivity [7].
The meandering shape ofthe heater is deposited on a surface area of 1 cm2• In appendix
e
the maximum heat loss was determined to be 5 Watts at an almost uniform temperature of 80oe
of the structure. In the conducted experiments the temperature directly after the heater is below 50oe.
Furthermore a large part of the structure will be even at a lower temperature, which decreases the losses even further. The power removed by forced water mierocharm el cooling will be within 2% of the input power in these experiments.The pressure drop was measured as a function of the cooling flow as shown in tigure 2-12.
For conventionally sized channels the pressure drop over the channels can be calculated by
!::J>
=
p. v2 . f. L (2.15)2
DH
where öP is the pressure drop (N/m2), p the density ofthe fluid (kg/m3), v the mean velocity (m/s), fthe friction factor(-), 1 the length ofthe channel (m) and Dh the hydraulic diameter (m). For laminar flow the friction factor is independent on the roughness of the channel, and can be calculated by
f =
64 (2.16)Re
The results of equation 2.15 with a constant viscosity of 0,001Ns/m2 arealso depicted in figure 2-12, together with the pressure drop calculated by Flotherm.
The theoretica! pressure drop is linear while both the Flotherm simulations and
experimental values are showing a non-linear behaviour. At low flowrates the theoretica!
pressure drop values are in excess of the Flotherm simulations and experimental val u es.
This is caused by the fact that at low flow rates the outlet temperature of the water is higher (for flow rate of 0,1 Vmin the outlet temperature is 42
oe).
As can be seen in appendix E the viscosity of water is dependent on temperature and drops with increasing temperature. This will result in a lower pressure drop as calculated with equation 2.15, where a constant viscosity, evaluated at the inlet temperature (20 °C), is used.At flow rates in excess of 0.4 1/min the experimental value becomes higher than the pressure drop calculated with equation 2.15. The hydraulic entrance length becomes higher with increasing flow rate, and will increase the pressure drop. Furthermore the entrance and exit effects will increase the pressure drop.
The Flotherm calculations are in very good agreement with experiments for flow rates up to 0.8 Vmin. Afterwards the difference increases. At a flow rate of 1 Vmin the measured pressure drop is 15 % higher than the calculated pressure drop. This effect may be caused by a transition from laminar to turbulent flow.
3
Figure 2-12 Pressure drop as ajunetion of water flow rate
The Reynolds number at a flow rate of 0.8 Vmin is 900. For a conventionally sized channel the flow becomes turbulent at Re~ 2300. However it is seen in earlier
experiments by Peng et al. [10] that flow transition between laminar and turbulent flow occurred for Re ~ 200-700.
The cooling capacity is determined by calculating the thermal resistance of the microchannel structure, as explained in sectien 2.3.2. In tigure 2-13 the thermal resistance, determined with the heater resistance, is shown as a function of the cooling flow.
0,4
I•·
experiment; old heater calib.111 experiment; new heater cal ib. : 1
1oflotherm; multichannel I
Figure 2-13 Rthheater as ajunetion of water flow rate; Q=105 W
The thermal resistance of the microchannel structure shows a decreasing value for increasing flow, which is caused by the decreasing capacitive resistance. In figure 2-13 two series of points are presented as experimental data. The heater calibration, as explained in appendix D, was performed before the measurements were done. It was observed after the experiments that the heater had become partially oxidized. Since this can influence the electrical resistance, the heater calibration was performed again. The calibration showed that the electrical resistance was altered indeed. Using the new calibration data results in a significantly higher thermal resistance. However since it is unknown whether the oxidization has occurred before, during, or after the experiments, the true value can lie anywhere between the two series of points. This makes the thermal resistance data obtained with the heater temperature unreliable.
Therefore the thermal resistance will be determined using the thermocouple temperature Tj-out instead of the heater temperature from this point onwards. The results using
equation 2.8 as definition for the thermal resistance are shown in figure 2-14. In this figure the Flotherm calculations are represented by a mean value with error bars. This is done because the value of Rth Tj-out is determined by taking the mean temperature of a surface of 2x2 mm, since the exact position of the thermocouple is unknown. The maximum value ofthe error barrepresents the value attained with the maximum temperature on the 2x2mm surface. The minimum value represents the value attained with the minimum temperature on the 2x2 mm surface.
The temperature difference is measured with calibrated thermocouples. The calibration showed a maximum deviation of0.12 K between the thermocouple reading ofTmand the other thermocouples for the complete measuring range. The standard deviation on the reading of all thermocouples is about 0.20 K. The overall error ofthe temperature measurement difference is expected to be within 0.5 K. As pointed out in the beginning ofthis section the heat losses are small (within 2% of input power). This results in an overall error in the thermal resistance in the order of 0.01 KIW.
\
Figure 2-14 RthTj-out as ajunetion of water flow rate; Q=105 W
*'experiment I
A flotherm; multichannel :
The results in figure 2-14 show the sametrend as figure 2-13. The thermal resistance decreases due to the decreasing value of the capacitive resistance. It is seen that the values calculated with Plotherm are in good agreement with the experimentally determined values. From a flow rate of 0.8 1/min the thermal resistance calculated by Plotherm is in excess of the experimental determined resistance. However the difference is smalland within the uncertainty ofthe measurement (0.01 KIW). The thermal
measurements do not give a clear indication of flow transition.