Table of Contents1.ABSTRACT:22.TITLE: FREE AND FORCED CONVECTION32.1.OBJECTVE:32.2.FREE AND FORCED CONVECTION HEAT EXCHANGER32.3.INTRODUCTION32.4.DESCRIPTION:32.5.EXPERIMENTAL CAPABILITIES42.6.THEORY42.6.1.FREE CONVECTION42.6.2.FORCED CONVECTION:4EXTENDED SURFACE52.6.3.TEMPERATURE DISTRIBUTION ALONG EXTENDED SURFACE62.6.4.HORIZONTAL AND VERTICAL FLAT PLANE IN FREE CONVECTION:62.7.PROCEDURE72.8.EXPERIMENT:72.8.1.OBJECTIVE72.8.2.PROCEDURE:84.1.1.OBSERVATIONS:82.8.4. Graph83 .Experiment # 593.1.Objectives93.2.Equipment Set-Up93.3.Equipment setup103.4.Procedure103.5Observations103.6.Graph11CONCLUSION:11

ABSTRACT:The gist of this experiment is to study one of the modes of heat transfer convection. Convection has two types; forced and free convection. In former we study the relation between power and surface temperature. Whereas later deals with the study of relationship between fan velocity and temperature. Both of these relations are represented graphically to analyze and interpret the data.

1. TITLE: FREE AND FORCED CONVECTION1.1. OBJECTVE:The main objective of this experiment is to study the characteristics of free and forced convection.1.2. FREE AND FORCED CONVECTION HEAT EXCHANGER1.3. INTRODUCTIONHeat transfer by simultaneous conduction and convection, whether free or forced, forms the basis of most industrial heat exchangers and related equipment. The measurement and prediction of heat transfer coefficients for such circumstances is achieved in the EES Free & Forced Convection Heat Exchanger by studying the temperature profiles and heat flux in an air duct with associated flat and extended transfer surfaces. The vertical duct is so constructed that the air temperature and velocity can be readily measured, and a variety of plug-in modules of heated solid surfaces of known dimensions can be presented to the air stream for detailed study. A fan situated at the top of the duct provides the air stream for forced convection experiments.A Control Panel contains temperature measurement, power control, and fan speed control circuits with appropriate instrumentation. Temperature measurement, to a resolution of 1oC is effected using Thermocouple sensors with direct digital read-out in 0C.Air velocity is measured with portable anemometer mounted on the duct.The power control circuit provides a continuously variable, electrical output of 0-100 watts with a direct read-out in watts.Using the instrumentation provided, free and forced convective heat transfer coefficients may be determined for: -1. A flat surface2. An array of cylinders (pinned heat sink)3. An array of fins (finned heat sink)Each module may be used independently on the bench, to establish free convection coefficients for horizontal orientation.The apparatus is fully self-contained.1.4. DESCRIPTION:The EES Free & Forced Convection Heat Exchanger consists of a vertical rectangular duct supported by stand and a control panel. A flat plate pinned or finned exchanger may be installed in the duct and secured by a quick-release catch on each side. Each exchanger incorporates an electric heating element with thermostatic protection against overheating. The temperature at the base of each exchanger is monitored by a thermistor sensor with connecting lead.The exchanger in use may be viewed through an acrylic window in the wall of the duct.An upward flow of air may be generated in the duct with a variable speed fan mounted at the top of rectangular duct.Air velocity in the duct, whether natural or forced, is indicated on a portable anemometer held in a bracket on the duct wall. The anemometer sensor is inserted through the wall of the duct.Temperature sensors are provided for the measurement of the in-going and out-going air temperatures together with surface temperatures of exchanger pins and fins. A digital readout indicates the temperature using a thermistor probe connected to a flexible lead.These temperatures are determined by inserting the probe through access holes in the duct wall.A control panel incorporates a variable power regulator with a digital readout to control and indicate power supplied to the exchanger on test. The exchanger is connected to the panel via the supply lead. A variable low voltage D.C. supply is provided for the fan via the supply lead.Power is supplied to the equipment via a supply lead connected to control panel.1.5. EXPERIMENTAL CAPABILITIES1. Demonstration of the relationship between input and surface temperature in free convection.1. Demonstration of the relationship between input and surface temperature in forced convection.1. Demonstration of the use of extended surfaces to improve heat transfer from the surface.1. To determine the temperature distribution along an extended surface.1. Comparison of a horizontal and vertical flat plate in free convection.1.6. THEORY1.6.1. FREE CONVECTION

A heated surface dissipates heat primarily through a process called convection. Heat is also dissipated by conduction and radiation, however these effects are not considered in this experiment. Air in contact with the hot surface is heated by the surface and rises due to a reduction in density. The heated air is replaced by cooler air which is in turn heated by the surface and rises. This process is called free convection.The hotter the temperature of the surface, the greater the convective currents and more heat (power) will be dissipated. If more power is supplied to a surface, the temperature of the surface must rise to dissipate this power.

Watts

Typical graph of power against surface temperature tH - tA

1.6.2. FORCED CONVECTION:In free convection the heat transfer rate from the surface is limited by the small movements of air generated by this heat. More heat is transferred if the air velocity is increased over the heated surface. This process of assisting the movement of air over the heated surface is called Forced Convection. Therefore a heated surface experiencing forced convection will have a lower surface temperature than that of the same surface in free convection, for the same power input.

Velocitym/stH tA

Typical graph of air velocity against surface temperatureEXTENDED SURFACE

Heat transfer from an object can be improved by increasing the surface area in contact with the air. In practice it may be difficult to increase the size of the body to suit. In these circumstances the surface area in contact with the air may be increased by adding fins or pins normal to the surface. These features are called extended surfaces. A typical example is the use of fins on the cylinder and head of an air cooled petrol engine. The effect of extended surfaces can be demonstrated by comparing finned and pinned surfaces with a flat plate under the same conditions of power input and airflow.

Air Velocity m/s

tH - tA oC

Typical graph of air velocity against surface temperature

1.6.3. TEMPERATURE DISTRIBUTION ALONG EXTENDED SURFACE

For a heat exchanger to be 100% efficient, the whole of the extended surface must be at the same temperature as the backplane. In practice, this cannot occur because the flow of heat along the pins or fins by conduction causes a temperature gradient to occur. The greater this gradient, the less efficient the heat exchanger will be.The efficiency of the heat exchanger must not be confused with the effect of a change in surface area e.g. comparing pinned and finned. For example, if the pinned and finned heat exchangers supplied with the equipment are compared, the pin is more efficient than the fin (slightly smaller temperature gradient) but the finned exchanger has a significantly larger surface area than the pinned exchanger and can dissipate more heat for the same surface temperature.

FINNEDPINNED

Figure 4: Graph of surface temperature against distance from back plate.1.6.4. HORIZONTAL AND VERTICAL FLAT PLANE IN FREE CONVECTION:

When a temperature difference is established between a wall and a stationary fluid, the fluid adjacent to the wall will move upward if the wail temperature is higher than that of the fluid and downward if the wall temperature is lower. Density gradients are set up in the fluid resulting in buoyancy forces and free convective currents. The rate of heat transfer depends mainly on the fluid motion. The orientation of the plate affects this movement of air. A horizontal plate restricts the movement of air and reduces the heat transfer. The same plate mounted vertically will give improved heat transfer.1.7. PROCEDUREStart-up Procedures

1. Connect the mains input power supply plug to a nearest single-phase electrical supply of 240VAC/50Hz.1. Turn the heater power control knob and fan speed control knob fully anti-clockwise.1. Connect the fan supply lead to the socket at the side of the control panel.1. Connect the temperature probe lead to the socket beneath the control panel.1. Clamp the flat plate heat exchanger into the duct using the two toggle clamps and connect the heater power supply lead to the socket on the cover.1. Connect the plate temperature (TH) connector to the socket on the heat exchanger.1. Place the meter into the bracket situated on the side of the duct.1. Ensure that the sensor hole is aligned with the direction of the airflow when inserting the probe through the wall of the duct. 1. Switch on the main power.1. Check that the L.E.D. temperature meter and Volt/Ampere Meter are illuminated. Check that the temperature meter indicates ambient temperature.1. Increase the heater power in the exchanger by rotating the power control knob clockwise. The power supplied to the exchanger should be shown in watts on the meter.1. Switch on the fan and increase the speed by rotating the fan speed control knob clockwise.1. Observe that the air velocity is indicated on the indicator.1. Check that the plate temperature (TH) increases.1. Set the heater power control and fan speed control knob to minimum.1. Now you are ready for the following experiments.Shut Down Procedures

1. Switch off the heater and turn the power control knob fully anti-clockwise. Set the fan speed control knob to maximum to cool down the hot plate heat exchanger.1. Turn off the main power supply after plate heat exchanger has cooled down to room temperature.1.8. EXPERIMENT:1.8.1. OBJECTIVETo demonstrate the relationship between power input and surface temperature in free convection.Equipment Set-Up:

1.8.2. PROCEDURE:1. Remove the fan assembly from the top of the duct.2. Place the finned heat exchanger into the test duct.2. Set the heater power control to 20 Watts (clockwise).3. Allow sufficient time to achieve steady state conditions before noting the heated plate temperature (tH) and ambient temperature (tA) into the table below.5. Repeat this procedure at 40, 60 and 80 Watts. 6. Plot a graph of power against temperature (tH-tA3.1.1. OBSERVATIONS:

Input Power WattsPlate Temp (tH) CAmbient Temp (tA)CtH tA C

2333258

58352510

2.8.4. GraphTemperature(x axis) V/S Power(Y axis).

3 .Experiment # 53.1.Objectives To demonstrate the relationship between fan velocity and surface temperature in forced convection.

3.2.Equipment Set-Up Plate Sensor Heater Probe Sensor Air Flow Wattmeter (Q) Temperature Indicator

3.3.Equipment setup

3.4.Procedure1. Place the fan assembly on to the top of the duct. 2. Place the finned heat exchanger into the duct. 3. Set the heater power control to 50 Watts (clockwise). Allow sufficient time to achieve steady state conditions before noting the heated plate temperature (tH) and the ambient temperature (tA). 4. Set the fan speed control to give a reading of 0.5m/s on the thermal anemometer, allow sufficient time to achieve steady state conditions. Record heated plate temperature (tH) and ambient temperature (tA). 5. Repeat this procedure by setting the fan speed control to give 1.0m/s and 1.5m/s. 6. Plot a graph of air velocity against temperature. ( tH tA)

Power input = 20 Watts 3.5Observations

Air Velocitym/sPlate Temp (tH)CAmbient Temp (tA)CtH tA C

1.331256

1.930255

2.729254

10

3.6.GraphTemperature(x axis) V/S Fan Velocity(Y axis) .

4.CONCLUSION:After conducting the experiment it was concluded that convection is a mode of heat transfer which is clearly observed by the difference in the temperature of the plate and the ambient temperature.Phenomenon of forced convection was also observed and it was concluded that as the velocity of the moving fluid increases the heat transfer rate increases and the temperature of the plate decreases due to this heat transfer. Graphical representation is used to analyze the data.

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