ENME 332 STUDIO: HEAT CONVECTION EXPERIMENT

Fall 2011

1209 Kim Building, Thermal Fluids Instructional Laboratory

The objective of this experiment is to measure the convection heat transfer coefficient for

a flat plate in a cross-flow of air and to compare the experimental results with the

correlations. This experiment is also intended to help students better understand external

flow heat transfer.

A schematic of the test rig in this experiment is shown Figure 1. A flat plate (12.7cm in

width and 15.2cm in length) with an internal cartridge heater is placed in an air flow

produced by a wind tunnel. The plate surface temperature Ts is measured by an attached

thermocouple. The upstream air temperature T∞ is assumed to be at the ambient room

temperature. The velocity of the air flowing over the plate is calculated using

measurements from a manometer (up to 100 mm H2O) and Bernoulli’s equation.

Figure 1: Schematic of the convection apparatus.

The heat transfer coefficient can be determined from Newton’s law of cooling:

q = h As (Ts - T∞)

where the power q is measured from the current and voltage across the heater, and Ts and

T∞ are temperature of the flat plate and the free stream air, respectively. These quantities

are known or can be measured. We will compare this experimental result with the

prediction from various correlations.

Experiment Instructions:

The lab group turns on the fan and then the cartridge heater. Record the plate

temperature as a function of time and indicate when steady state has been reached.

You need to answer the following questions in your lab report.

1) Plot the temperature Ts as function of time to see how it varies and to determine when

steady state is reached. Explain any oddities in your measurements.

2) Fill out the following table.

Run Voltage (V) Resistance

(Ω)

Manometer *

(mm H2O)

Air temperature

(°C)

Plate

temperature (°C)

1

3) What is the free stream air velocity? The air velocity can be calculated from the

Bernoulli Equation:

ZγVρ2

1pp 2

tot ++=

4) What is the power supplied to the heater?

5) What is the heat transfer coefficient h calculated from Newton’s law of cooling?

What is the corresponding Nu?

6) Evaluate Re and the Pr for your system condition. From your Re, what would you

expect your flow regime to be (i.e., would you expect your flow to be laminar or

turbulent)?

7) Compute Nu using the following correlation. How do the measured values of Nu

compare to this prediction?

Laminar Flow over Flat Plate, Constant Ts: = 0.664 / /

Discussion:

For discussion, provide two possible reasons to explain why the experimental Nu is

different than the Nu predicted from the correlation. As with the previous lab report,

support each of your reasons with discussion, equations, and whatever else is needed to

demonstrate that your reasons of choice could be a part of the difference you see between

your experimental and correlation values. For example, if your correlation Nu is higher

than your experimental Nu, each of the two reasons you choose must

mathematically/logically demonstrate that the correlation Nu is too high or the

experimental Nu is too low. You can consider the geometry of the physical system

compared to the correlation, the flow assumptions being made (in both systems), the

origin of the equations involved, or other factors that come to mind.

Report:

Each group should submit one report presenting their data, calculations, results, and

discussion. Submissions can be softcopy (PDF please) or hardcopy. Reports are due one

week after your lab was conducted (by end of day).