(145b) Some Very Easy and Important Laboratory Practices in Transport Phenomena

In this material it is intended to share the experience gained during more than ten years of proposing and teaching some laboratory practices of transport phenomena, having available a low budget, but motivated by the necessity of having them due the course requirements.

All the practices of laboratory should require observation, measurement and calculations, but due their nature, some ask additionally to the student 1) practical abilities such as to close or to open a valve at a specific time, to measure some property, etc), 2) to analyze the theory, to obtain mathematical models and to solve the resulting equations, in order to compare with the experimental data, 3) Some combinations of this two points. The practices described here are classified in some of these categories.

1.- Four practices of heat transfer.

a) Qualitative evidence of the different capacity of several materials to conduct heat.

In this practice, it is shown trough a very simple experiment how aluminum, bronze and iron present different thermal conductivity. Several small candles (as those used in birthday cakes) are placed along the length of a cylindrical bar; a burner is used to heat up one end of the bar. It is observed that as the heat advances the candles melt and fall at different times in each material because of the difference in conductivity for each bar.

b) Determination of the transient axial temperature profiles. Again, a burner is placed in one end of the bar, but now thermocouples are placed in different positions along its length to measure the axial temperature profiles at different times. In this way, curiosity is generated about looking for a mathematical model that describes the obtained experimental profiles.

c) Determination of the provided heat and measurement of the axial transient temperature profiles. In the former practices, the amount of provided heat was unknown. In this experiment, the heat flux is calculated by using a piece of equipment with an electrical resistance. The axial temperature profiles are measured at different times and the experiment is prolonged until steady state condition is achieved in order to illustrate the Fourier Law. The bars are insulated and the effect on axial temperature profiles is studyied. The different possible boundary conditions to be used in this system are also illustrated. Moreover, it is encouraged to find the solution of the partial differential equation contained in the mathematical model.

d) Determination of the continuity boundary conditions in the temperature profile. Two bars of different material are placed together in series (as a composite wall) and several thermocouples are placed in both bars. In one end, heat is provided and the transient temperature profiles are measured until reaching the steady state. In the position where the bars are joined, continuity in the temperature profiles and change in the slopes from one material to the other should be observed. This practice additionally illustrates technical problems and their solutions that normally arise during the experiment. From the experiment results, the students reinforce their knowledge and strengthen their character.

2.- Two practices of momentum transport.

a) Determination of the height profile of a liquid surface in a fixed cylindrical vessel with a spinning central bar. Water is placed in a cylindrical vessel (as a blender) equipped with a spinning central bar. The blender is turned on and the effect of the spinning velocity of the bar on the form of the liquid surface is studied. Pictures are taken and they are compared with the solution of the differential equation obtained from the momentum analysis.

b) Determination of the height profile of the liquid surface in a rotating container.

For this experiment, water is placed in a cylindrical container that rotates at different angular velocities, photographs are taken for different velocities and they are compared with the solution of the differential equation resulting from the momentum analysis. In this case, other boundary conditions are used, due the physical problem is different.

3.- Practice of mass transport.

It is intended to simulate a human stomach by dissolving a medicinal pill in a vessel containing water and a stirrer, which rotates at different angular velocities. The mass transfer and diffusion coefficients are determined.

In summary, facing the lack of economical resources and given the necessity of laboratory practices, very elemental and cheap equipments were constructed to illustrate the concepts and challenges involved in the analysis of heat, mass and momentum transfer problems. With these laboratory practices, the students could solve the resulting equations from the theoretical analysis, perform measurements and compare results