###### Workshop #12 - Introduction to Heat Transfer Coefficients

Read about heat transfer coefficients

A heat transfer coefficient is used to relate the heat flux between a surface and a fluid. For heat flux defined as positive from the surface

where Ts is the surface temperature and Tfluid is the temperature of the bulk fluid. If the temperature difference is zero, the value of h is undefined. Therefore, heat transfer has to be provided into or out of the surface for h to have meaning. If there is no heat transfer occurring naturally, a heater may be added to provide a heat flux. The temptation might be to only use the heater and assume that all of the heat flux goes into the air by convection. This is generally not a good assumption. Without the heat flux sensor it is difficult to know how much of the thermal energy from the heater actually leaves to the fluid and how much is absorbed by the surface material. The value of a heat flux sensor is that it directly measures the heat flux from the heater on the surface of the wall to the fluid.

Placing a heater behind the heat flux sensor creates an artificial “hot spot” where the sensor is, however. This disruption of the surface temperature is known to increase the heat transfer coefficient at the location of the sensor. This results in measurement errors that can be quite large. The following reference gives additional details. There is also guidance on using heat flux sensors in the two ASTM standards that are listed.

Diller, T. E., “Heat Flux Measurement,” Ch. 18, in Handbook of Measurement in Science and Engineering, Ed. M. Kutz, John Wiley & Sons, NY, 2013, pp. 629-659.

ASTM E2684-17, Standard Test Method for Measuring Heat Flux Using Surface-Mounted One-Dimensional Flat Gages. Ann. Book ASTM Standards, 15.03, 2017.

ASTM E2683-17, Standard Test Method for Measuring Heat Flux Using Flush-Mounted Insert Temperature-Gradient Gages. Ann. Book ASTM Standards, 15.03, 2017.

Read about heat transfer coefficients

Normally the heat flux through building walls is very small. Consequently, to measure the convective heat transfer coefficient on the outside of the wall, add the heater provided in the sensor kit to give a measurable heat flux. Place it between the heat flux sensor and the wall of a building. Put the side of the sensor with the thermocouple away from the building to get a positive heat flux when the heater is operating. Turn on the data acquisition for at least 10 seconds and then activate the heater. Let it run for a least 30 seconds. Then find the average values before and after turning on the heater.

1. Plot the temperatures, heat flux and heat transfer coefficient as a function of time. Attach to the workshop. Do the values of temperature and heat flux before the heater is turned on make sense?

2. Show the calculations for the heat transfer coefficients.

3. The heater increases the temperature of the sensor and the resulting convective heat flux, which makes it easier to determine the heat transfer coefficient. However, this increased surface temperature locally disrupts the thermal boundary layer, increasing the local heat transfer coefficient. Based on your measurements, how much does the h value increase for this case?

4. What happens to the heat transfer coefficient under transient conditions when the direction of the temperature difference changes from positive to negative?

5. What can you conclude about the limitations of heat transfer coefficients and how they are defined?