We’ve designed a series of 14 hands-on workshops for students to use as part of a heat transfer course. Using a kit that includes heat flux and temperature sensors run by any laptop computer makes this a true mobile lab for students. Real-time plots of heat flux enhance students’ interest as they measure and interpret real-world thermal events. Data is saved for analysis of basic course concepts. The kit provides an inexpensive laboratory experience without the costs and logistics of dedicated laboratories. A list of activities is included below and to the right. At the bottom of the page you will find details on each of our 14 workshops.
We’ve run hands-on workshops for over 500 mechanical engineering students at Virginia Tech and have seen enthusiastic student response. Increased student engagement in the course has been very encouraging, especially for those students who are usually not motivated by a heat transfer course. The results of the initial hands-on workshops sponsored by the NSF are documented in the following paper:
Cirenza, C. F., Diller, T. E., and Williams, C. B., “Hands-On workshops to Assist in Students’ Conceptual Understanding of Heat Transfer,” ASME Journal of Heat Transfer, Vol. 140, 2018, 092001, 10 pages.
This exercise allows the students a literal hands-on experience. Because the heat flux sensor allows direct measurement of transfer coefficients, students can use a transient energy balance with the usual exponential solutions for lumped capacitance models. Actual plots of both the temperature and heat flux helps them to see the correspondence between heat and temperature. They also get to “feel” the change in temperature of the aluminum with time to relate to the measured values. The match between the predictions and the actual data gives students a connection between the theory and the real world. The reason for the cloth is to slow the response to make it easier to observe.
Blowing on something to heat or cool it is common. Mounting the heat flux sensor on a heat sink allows measurement of the heat flux. Separate measurements of the surface temperature and air temperature with thermocouples allows calculation of the convection heat transfer coefficient. The h value is typically much larger than the h values from W8 because the air velocity is much larger than one can move their arm. The corresponding lung pressure from Bernoulli’s equation should give several inches of water pressure. This gives students a reference to compare their numbers.
Surface temperature and heat flux measurement are made on both sides of a pane of glass. Ideally this is done on either a cold or hot day so that there is heat transfer occurring into or out of the building. Students should draw a control volume around the system and show that the heat flux to the glass on the inside must equal the heat flux leaving on the outside. A plot of the temperature distribution is also required. This is to relate what they see in the textbook with what they can actually measure. The temperature across the glass is also calculated and compared to what is measured. However, it will typically be a much smaller difference than measured because of the poor contact of the bead thermocouple with the surface. The surprising conclusion is that it is much harder to accurately measure temperature than heat flux in this case.
This is a repeat of Workshop 8, but now with a wet cloth to provide mass transfer in addition to convective heat transfer. The Lewis relation is used to relate the heat transfer coefficient and the mass transfer coefficient. Students then calculate the simultaneous heat and mass transfer. The relative humidity of the air is not directly measured, but is inferred by the total energy transfer from the surface. With the wet surface, the sensor temperature will be lower, which decreases the heat transfer. The energy transfer from the evaporation should dominate the overall energy transfer from the surface.
Workshops 8 and 9 illustrated simple cases where measurements of heat flux can be used to find heat transfer coefficients between fluids and surfaces. This is easy when there is already substantial heat transfer occurring at the surface. Heaters can be used to artificially create heat flux at a surface, although it usually takes a long time to reach steady-state conditions and challenging to account for losses. The heat flux sensor can avoid the steady-state criterion by directly measuring the heat flux to the fluid. This workshop uses the heater supplied to calculate heat transfer coefficients as the surface temperature changes. It illustrates the importance of how the value of h is defined and how the apparent value can change with varying conditions. It also shows how changing the surface temperature locally where the heater is can increase the apparent heat transfer coefficient. This has sometimes been called the “heat island effect”.
This lab is best done with a radiation source that is provided to the students. The power required is more than available from their computers. For a six-inch square plate with an electric resistance heater on the back requires at least 10 watts of power to reach 80° or 90°C, which makes it easy for the students to feel the radiation from the plate. A 1/8 inch thick aluminum plate placed this on some type of thermal insulation works well. Use some flat black spray paint to cover one-half of the plate. Polish the other half of the plate with steel wool or the equivalent. This should provide a very measurable difference in radiation heat flux that can both be felt by the students and measured with the heat flux gage. With some simplifying assumptions the emissivity of the bare metal surface can be measured. The workshop is very useful in helping the students appreciate the effect of surface emissivity on radiation exchange. It makes it seem real for them.
This is one of the student favorites where they get to use their creativity to measure the heat flux in their own problem. They give a short report on their problem and their results.