CONSTRUCTION

Local surface heat flux and temperature measurements are provided by an array of platinum resistance heater elements deposited on a quartz wafer in a serpentine pattern. Typically, ninety six of these heaters are arranged in a square array. Heater arrays of different size can be made. The array in Figure 1 has elements 0.26 mm x 0.26 mm in size, each of which has a nominal resistance of 1000 ohms and a nominal temperature coefficient of resistance of 0.002 °C-1. The lines in the heaters are 5 microns wide on a 5 micron spacing. Since the heater lines only cover half the quartz, pictures can be obtained through the quartz wafer. Aluminum or gold leads snake between the heaters and are used to make connections to them. Other square arrays with elements 0.1 mm x 0.1 mm and 0.7 mm x 0.7 mm as well as linear arrays (1 x 60) have been fabricated and operated.

A chip containing the heater array is mounted on a Pin Grid Array (PGA) which is then mounted on a Printed Circuit Board (PCB) so that connection can be made to the feedback loops. Wire bonds are used to make electrical connections between the chip and the PGA.

 

Figure 1: Heater array

 

HEATER OPERATION

We typically operate in constant temperature mode by using electronic feedback circuits to keep each heater at a constant temperature (constant resistance), and measuring the time varying voltage across each heater. A schematic of a feedback circuit is shown on Figure 2. The heater is represented by a resistance Rh which is one resistance in a Wheatstone bridge. The op-amp (LTC1150) measures the imbalance in the bridge and outputs a voltage to bring the left and right sides of the bridge into balance (i.e., V+=V-). The voltage Vout is sampled, allowing the heater power to be determined. The resistance of the heater Rh can be changed by changing the wiper position of a digital potentiometer (DS1260). The large 200 Kohm resistance at the top of the bridge is used to provide a small trickle current so the op-amp can measure imbalances in the bridge even if the it does not output a voltage. The other resistances are chosen such that regulation of the heaters can be achieved between room temperature and a certain maximum temperature. Because of the finite resolution of the digital pot (typically 512 positions), a larger temperature range results in coarser temperature steps.

Because all the heaters in the array are at the same temperature, heat conduction between adjacent heaters is negligible. There is conduction from each heater element to the surrounding quartz substrate, but this can be measured and subtracted from the total power supplied to the heater element, enabling the power supplied to the fluid to be determined.

The heaters can also be operated in constant heat flux mode. A voltage is supplied across the heaters and the current through them is measured, allowing the resistance (temperature) vs. time to be measured.

 

 

 

Figure 2: Circuit schematic (click for larger image).

HEATER PERFORMANCE

An example of the heater performance is given in Figure 3. FC-72 (Tsat=56 °C) was boiling on a heater array. The time resolved heat transfer from a specific heater in the array at critical heat flux is shown on Figure 3. The frequency response of the heaters and circuit together is very high (~15 kHz), much faster than transients typically associated with most phase change processes. Regions of low heat transfer can be associated with vapor covering the heater.

Figure 3: CHF data. Click for larger image.

 

CALIBRATION

Calibration consists of placing the heater array in a know thermal environment and determining the digital pot positions required to cause the op-amp to just start regulating. First, all of the digital pots are set to zero (smallest resistance), and it is verified that all of the heaters are not regulating. The digital pot position for each heater is incremented until the value at which Vout begins to increase (Figure 4)–this value is noted for each heater.

 

Figure 4: Calibration example. Click for larger image

Two early papers that describes application of this technique to boiling are

1). Rule, T.D. and Kim, J., “Heat Transfer Behavior on Small Horizontal Heaters During Pool Boiling of FC-72", Journal of Heat Transfer, Vol. 121, No. 2, May, 1999, pp. 386-393.


2). Bae, S., Kim, M.H., and Kim, J.,, “Improved Technique to Measure Time and Space Resolved Heat Transfer under Single Bubbles during Saturated Pool Boiling of FC-72", Experimental Heat Transfer, Vol. 12, No. 3, 1999, pp. 265-278.

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