Abstract
The purpose of this report is to present experimental data that demonstrate the relationship between air velocity and surface temperature for a cylinder subjected to forced convection.
This experiment will also determine the effects of forced convection on heat transfer from the surface of a cylinder at varying air velocities and surface temperatures. The forced convection theory is based on the same principle of moving air over a surface to increase the heat transfer rate from a surface. Therefore a surface subjected to forced convection will have a lower surface temperature than the same surface subjected to free convection, for the same power input.
The force convection will be provided by the air compressor on the apparatus. The heat transfer will also be accomplished by means of radiation heat transfer as heat will be absorbed by the surroundings. The experiment is setup using:
- HT10X Heat Transfer Service Unit
- HT14 Combined Convection and Radiation Accessory
- IFD3 PC Interface Console
- Compatible PC
The air compressor will be operated at fully open and the air velocity will be systemically reduced back to zero or a natural convection state.
The cylinder will be heated to a prescribed temperature corresponding to a particular voltage (14 volts). With the flow stream fully open the surface temperature of the cylinder will be read at the T10 thermocouple. The T9 thermocouple will be place upstream of the heated cylinder and measure the temperature of the air. There will be a great difference in temperature between the two thermocouples. As the flow across the surface of the cylinder is slowed the surface temperature will increase.
Introduction
The experiment is setup to use the heat source supplied from the electrical outlet to the apparatus W); the voltage entered (14v) will heat the cylinder and the corresponding value will be read from the apparatus. The velocity of the compressor will be full open to 6 m/s. Once the temperature has stabilized, the values at T9 and T10 will be recorded. Reduce the compressor velocity by an increment of 1 m/s and wait for the temperature stabilization; record stable temperatures at T9 and T10.
Reduce the compressor velocity by 1 m/s until the compressor has stop, then record the respective temperatures. Calculate the Total heat loss from the cylinder using where Qf is the heat loss due to forced convection and Qr is the heat loss due to radiation. The formulas for convective heat loss and radiation heat loss respectively, where hf is the convective heat transfer coefficient, hr is the radiation heat transfer coefficient. Ts is the surface temperature, Ta is the temperature of the surrounding. heat transfer coefficients, and As is the surface area of the cylinder given by where D is the diameter of the cylinder and L is the length. f is can be calculated. An empirical formula can be used to calculate the value for (Nusselt number) Num as follows: From SW Churchill and M Bernstein “A Correlating Equation for Forced Convection from Gases and Liquids to a Circular cylinder in cross-flow”. Journal of Heat Transfer, 99:300-306 (1977). The kf, Pr, will be taken from empirical data Incropera/DeWitt [2] and the Reynolds number (Re) is calculated using [pic] where V is the velocity of the air, D the diameter of the cylinder and ? is the kinematic viscosity of the air. The theoretical results will be calculated and compared with the experimental values.
The theoretical calculations for convection heat transfer coefficient will be determined by the Newton Law of cooling This value is based on the conditions in the boundary layer, which is influenced by the surface geometry (of the cylinder), the nature of the fluid in motion and an assortment of fluid thermodynamic and transport properties. The Experimental Nusselt number will be calculated using the empirical correlation for engineering calculations for overall average conditions.
Analysis and Discussion
The data for the total heat and the heat transfer rate were relatively high and may be attributed to a system malfunction, not enough data point or an assortment of variables. These results may be considered acceptable but further analysis would be required. The data also reveals that as the velocity of the air moving over the cylinder decreases the percent error between the heat that is supplied to the system (theoretical) and the experimentally calculated heat increases significantly. With constant voltage supplied to the system any drop in velocity will affect the rate at which heat is transferred from the cylinder. The percent of heat transfer due to convention is significantly higher at higher velocities. The radiation heat transfer was larger at the free convection state by a factor of 60%.
The heat transfer coefficient for the radiation is significantly lower than the heat transfer coefficient of friction. This was expected in the experiment as the heat losses due to radiation were meant to be very low. Due to this however the temperature rises because of the low heat loss due to radiation. The theoretical values of the Nusselt Number are very close to the experimental values although the small differences can be due to experimental errors such as human error and lack of accuracy from the equipment. The errors between the theoretical and experimental values of the Nusselt numbers ranged between 20% to 30% which is acceptable given the errors mentioned above.
We learn about the practical aspects of heat transfer due to convection from this experiment. Though the results were different from the theoretical results we saw that the fundamentals remained the same. The velocity of the air passing through the heated cylinder is directly proportional to the heat loss from the cylinder. This is shown in figure 4 where the convection coefficient is also directly proportional to the velocity of the air. It can also be seen in figure 2 as the temperature of the surface of the cylinder decreases as we increase the velocity of the air. This decrease in temperature is the result of more convective heat loss from the cylinder.
This graph in figure 2 also shows us that change in temperature at velocities between 0m/s and 3m/s is more drastic than the change in temperature from 3m/s to 6m/s. At the higher velocities the air flow does not affect the heat loss due to convection as it seems to level out after 3m/s. This means that the convective heat losses can only reduce the cylinder temperature till a certain point, after this point an increase in the velocity of the air will not affect heat losses as much. We saw from the other groups that a change in velocity resulted in different temperatures which in turn caused the heat losses to change at these voltages. The heat input was different as well due to this.
Conclusion and Recommendations
As a whole the experiment was a success, the errors that we encountered conducting the experiment and the errors in the results were all part of what was expected. As we were restricted for time the apparatus was not cooled down fully after each stage which could have resulted in inaccurate readings of the initial temperatures. As a recommendation we feel that a clear tube made from a plastic or plexi-glass could be used so that the air flow can be seen in the tube. The temperature should also be measured at different points along the length of the tube so that the conductivity constants in the air will be more accurate. Adding a thermocouple at every 90 degrees on the surface of the cylinder will also enable us to get more accurate results by taking an average over the entire surface.
Measuring the temperature at higher air velocities, maybe up to 10m/s will show us if the effects of convection still cause a significant difference. I think that this can be added to the experiment. Making an application for the experiment will also make it more interesting and give us more understanding for the practical uses of performing such experiments.
