Heat Transfer Mobile Laboratory Descriptions

Sensor kits are available at *Fluxteq*

For more information on the kit, please see this 2017 news article.

Heat flux sensor

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.

*+*Workshop 1 - Sensor System Introduction

This is a simple exercise to help students understand heat transfer (or heat flux) as a physical quantity. It is designed for students to easily relate to their previous concepts, which in this case is caloric food intake. The usual value is This workshop provides an introduction to the mobile sensor system. There is a check that the system is working properly and that data is saved to an Excel file. The difference between temperature and heat flux is emphasized along with typical values that should be measured. Why does heat flux have a positive and negative direction, while temperature is simply a scalar value? In addition, the idea that heat transfer processes are usually transient and that real data has fluctuations.

*+*Workshop 2 - Body Metabolism

This is a simple exercise to help students understand heat transfer (or heat flux) as a physical quantity. It is designed for students to easily relate to their previous concepts, which in this case is caloric food intake. The usual value is around 2,000 calories per day. Also, this workshop has students perform simple measurements to acquaint students with the heat flux measurement system. Questions include probing the difference between heat flux and temperature at a surface. Exposed skin should give higher heat flux, but lower temperature than when it is covered by clothing because clothing adds additional thermal insulation.

*+*Workshop 3 - Fins

An aluminum fin is included in the system kit. When combined with the heater and the heat flux sensor, the heat flux from the fin can be measured directly. This then allows the fin efficiency to be calculated based on the surface area of the fin. It can be compared with the theory. Most importantly, however, is the opportunity for the students to actually feel the temperature decrease from the base to the tip of the fin to understand how a fin works.

*+*Workshop 4 - Transient Lumped Capacitance MethodThis 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.

*+*Workshop 5 - Thermal Resistance

This exercise combines the topic of transient heat transfer in semi-infinite media with thermal resistance. The thermal conductivity of concrete and carpet are different by over an order of magnitude. This has a huge effect on the resulting heat transfer, but much less on the change of temperature. The heat transfer is much higher as time increases in the concrete than the carpet. The result is that the concrete “feels” much colder than the carpet, even though the initial temperature of the concrete and carpet are essentially the same. Alternatively, it can be viewed that the thermal resistance is much lower for the concrete than for the carpet. The higher thermal resistance of the carpet results in much lower heat transfer, but a larger temperature change than for the concrete. Consequently, the carpet feels much warmer to the feet than a tile floor.

*+*Workshop 6 - Heat Sinks and Energy Balances

Heat transfer is initiated by turning on the heater. Two different heat sinks are evaluated. One is a piece of metal while the other is carpet (an insulator). The temperature and heat flux response is evaluated for each by moving the heater from one side to the other of the heat flux sensor. This shows that one direction the heat flux is positive and the other it is negative. The sign of the heat flux is important, unlike temperature, which has no direction. Also, the quality of the sink and source determines the magnitude of the heat flux. The metal is a much better heat sink. Therefore, the heat flux is higher and the temperature change is lower. Students are encouraged to draw the system with an energy balance and heat fluxes marked to help visualize and to practice obtaining the corresponding equations.

*+*Workshop 7 - Relative Thermal Resistance of Cloth

This workshop uses the same technique as Workshop 4 with the hand providing the heat source to a low conductivity (carpet) and a high conductivity (metal) heat sink at room temperature. It emphasizes the concept that the importance of adding thermal resistance (in this case a piece of cloth) is dependent on the thermal resistance already present. Within the measurement uncertainty of 5%, the cloth should have no effect on the carpet, but 40% or 50% on the metal. The students should also note a big difference in the perceived temperature of the metal with and without cloth. This is the same effect as sitting on metal bleachers in shorts versus pants. The application also extends to heat exchangers when considering the effect of fouling. Fouling has a much greater effect when the overall heat transfer coefficient is high.

*+*Workshop 8 - Fluid Convection

Fluid convection is normally characterized by a convective heat transfer coefficient, h. To calculate h the heat flux along with the surface and fluid temperatures are required. These are provided by the heat flux sensor and thermocouple readings from the DAQ. Three different measurements are explored using the heat transfer from the wrist. The same characteristic length is required for both the Reynolds and Nusselt numbers. A correlation between the Reynolds and Nusselt numbers allows a calculation of the apparent velocity of the arm. The students try to maximize how fast they can move their arm.

*+*Workshop 9 - Jet Impingement External FlowBlowing 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.

*+*Workshop 10 - Glass Window ConductionSurface 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.

*+*Workshop 11 - Evaporation - Simultaneous Heat and Mass TransferThis 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.

*+*Workshop 12 - Heat Transfer CoefficientsWorkshops 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”.

*+*Workshop 13 - Gray Body RadiationThis 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.

*+*Workshop 14 - Make Their Own ProblemThis 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.

Workshop 14 details

Sample results are available upon request from Prof. T.E. Diller