Experiment Example

Background and Introduction to Experiment

Heat transfer enhancement due to various rib configurations was considered in a square channel by Han et al. [3]. The objective of this study was to evaluate the thermal performance of various rib turbulators in a square channel. The thermal performance of each rib configuration was evaluated through the measurement of both the heat transfer enhancement and the associated frictional losses. Regional heat transfer coefficients, and thus regional heat transfer enhancement, were obtained using copper plates and heaters.

Figure 5.4 shows the square test channel used in this study by Han et al. [3]. As the figure indicates the length of the square channel was divided into 10 isolated sections. The copper plates were in direct contact with the cooling air passing through the channel. Foil heaters were placed on the back of the copper plates. Finally, the copper plates and foil heaters were encased in wooden plates (to minimize the stray heat loss from the heaters away from the copper).

The length of the heated test section was 101.6 cm, and the cross section was 5.08 cm by 5.08 cm. Each of the forty copper plates was 5.08 cm wide by 10.16 cm long, and they were separated by 0.159 cm thick wooden strips. A blind hole is drilled on the back of each copper plate, and a single 24 AWG, type-T thermocouple was glued in each hole using high temperature, high conductivity epoxy. These 40 thermocouples measuring the regional wall temperatures were connected

FIGURE 5.4 Experimental setup for measurement of regional heat transfer coefficients.

to a Fluke 2285 В data logger for real-time monitoring of the channel temperatures. Thermocouples were also placed in the mainstream flow at the inlet and the outlet of the channel to measure the bulk air temperature at these locations.

Four foil heaters (one for each wall) were constructed from a 0.001-inch-thick stainless steel foil. Each heater was 5.08 cm wide by Ю1.6 cm long to cover one wall of the square channel. Lead wires were soldered to opposite ends of the heater, and power to each of the four heaters was controlled independently through four separate variable transformers. The foil heaters were attached between the copper plates and wood using double sided tape.

While this channel was used to study many turbulator configurations over a w'ide range of Reynolds numbers, only one case is considered in this example: the channel with 45° V-shaped ribs. Data collected from a representative Reynolds number (approximately 30,000) is show'n. Furthermore, the required steps to properly reduce the raw data for this case are provided.

Mass Flow Rate Calculation

Prior to beginning the experiment, it is necessary to calculate the required coolant mass flow rate. With the desired Reynolds number of 30,000, it is possible to determine the required flow rate.

After evaluating the viscosity of air at the channel’s bulk inlet temperature, the required mass flow rate, m, of air can be calculated from Equation (5.19). Care must be taken by the experimentalist to ensure that correct characteristic length is chosen for the experiment. In this case, the Reynolds number is defined based on the hydraulic diameter of the channel.

With the square channel described above, the required mass flow rate was determined by combining Equations (5.19) and (5.20) and rearranging the terms.

While 0.0273 kg/s is the target mass flow rate for Re = 30,000, it is difficult to meter the flow rate precisely to obtain flow; therefore, it is expected that the actual Reynolds number will vary slightly from the target number.

Heat Loss Calibration (Raw Data)

Although the test section has been constructed, so the structural components have relatively low thermal conductivities, stray heat losses have not been completely mitigated. Therefore, it is necessary to determine the magnitude of the heat loss. The heat losses were determined for the present channel from a “no flow” experiment. During this no flow experiment, the square channel was filled with a low conductivity, fiberglass insulation. Power was supplied to the foil heaters, and the walls were heated. Two different power settings were applied to the heaters to generate two sets of wall temperatures. These two sets of temperatures were chosen so one set was below the anticipated wall temperature during the actual test, and the second set was above the expected wall temperature. The two sets of temperatures bracketing the actual wall temperature make it possible to interpolate the heat loss during the actual test.

Due to the symmetry of the current channel, heat loss data is provided for one wall of the duct, and this data is assumed to be applicable for all four walls (Table 5.8). Upon reaching steady-state at the two different power settings, the resistance of the heaters was measured and combined with the measured voltage. These resistance and voltage measurements are shown in Table 5.9.

TABLE 5.8

Temperature Distributions Acquired through the Heat Loss Calibration

TABLE 5.9

Voltage and Resistance Measurements from Heat Loss Calibration