FREE & FORCED CONVECTION HEAT TRANSFER

Transcription

1 FREE & FORCED CONVECION HEA RANSFER Objective he objective of thi experiment are: (1) o demontrate the relationhip between power input and urface temperature in free convection (2) o demontrate the relationhip between power input and urface temperature in forced convection 1. Equipment Decription 1.1. Introduction Heat tranfer by imultaneou conduction and convection, whether free or forced, form the bai of mot indutrial heat exchanger and related equipment. he meaurement and prediction of heat tranfer coefficient for uch circumtance by tudying the temperature profile and heat flux in an air duct with aociated flat and extended tranfer urface. he vertical duct i o contructed that the air temperature and velocity can be readily meaured, and a variety of plug-in module of heated olid urface of known dimenion can be preented to the air tream for detailed tudy. A fan ituated at the top of the duct provide the air tream for forced convection experiment. A Control Panel contain temperature meaurement, power control, and fan peed control circuit with appropriate intrumentation. emperature meaurement, to a reolution of 0.1 i effected uing PD enor with direct digital read-out in o C. Air velocity i meaured with portable anemometer mounted on the duct. he power control circuit provide a continuouly variable, electrical output of 0- watt with a direct read-out in watt. Uing the intrumentation provided, free and forced convective heat tranfer coefficient may be determined for: 1. A flat urface 2. An array of cylinder (pinned heat ink) 3. An array of fin (finned heat ink) Each module may be ued independently on the bench, to etablih free convection coefficient for horizontal orientation.

2 Detailed DESCRIPION he equipment conit of a vertical rectangular duct upported by tand and acontrol panel. A flat plate pinned or finned exchanger may be intalled in the duct and ecured by a quick-releae catch on each ide. Each exchanger incorporate an electric heating element with thermotatic protection againt overheating. he temperature at the bae of each exchanger i monitored by a thermitor enor with connecting lead. he exchanger in ue 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 peed fan mounted at the top of rectangular duct. Air velocity in the duct, whether natural or forced, i indicated on a portable anemometer held in a bracket on the duct wall. he anemometer enor inerted through the wall of the duct. emperature enor are provided for the meaurement of the in-going and outgoing air temperature together with urface temperature of exchanger pin and fin. A digital readout indicate the temperature uing a thermitor probe connected to a flexible lead. hee temperature are determined by inerting the probe through acce hole in the duct wall. A control panel incorporate a variable power regulator with a digital readout to control and indicate power upplied to the exchanger on tet. he exchanger i connected to the panel via the upply lead. A variable low voltage D.C. upply i provided for the fan via the upply lead. Power i upplied to the equipment via a upply lead connected to control panel heoretical Background Free Convection A heated urface diipate heat primarily through a proce called convection. Heat i alo diipated by conduction and radiation, however thee effect are not conidered in thi experiment. Air in contact with the hot urface i heated by the urface and rie due to a reduction in denity. he heated air i replaced by cooler air which i in turn heated by the urface and rie. hi proce i called free convection. he hotter the temperature of the urface, the greater the convective current and more heat (power) will be diipated. If more power i upplied to a urface, the temperature of the urface mut rie to diipate thi power. Depite the complexity of

3 3 convection, the rate of convection heat tranfer Q [W] i oberved to be proportional to the temperature difference, and i conveniently expreed by Newton law of cooling a Q= ha( ) where h i the convection heat tranfer coefficient [W/m 2 ] A i the urface area through which convection heat tranfer take place i the urface temperature i the temperature of the fluid ufficiently far from the urface or the ambient air temperature Forced Convection In free convection the heat tranfer rate from the urface i limited by the mall movement of air generated by thi heat. More heat i tranferred if the air velocity i increaed over the heated urface. hi proce of aiting the movement of air over the heated urface i called Forced Convection. herefore, a heated urface experiencing forced convection will have a lower urface temperature than that of the ame urface in tree convection, for the ame power input.

4 4 2. Experimental Procedure 2.1. Startup Procedure (1) Connect the main input power upply plug to a nearet ingle-phae electrical upply. (2) urn the heater power control knob and fan peed control knob fully anti-clockwie. (3) Connect the fan upply lead to the ocket at the ide of the control panel. (4) Connect the temperature probe lead to the ocket beneath the control panel. (5) Clamp the flat plate heat exchanger into the duct uing the two toggle clamp and connect the heater power upply lead to the ocket on the cover. (6) Connect the plate temperature (H) connector to the ocket on the heat exchanger. (7) Place the meter into the bracket ituated on the ide of the duct. hen, connect the cable to the connection beneath the control panel. (8) Enure that the enor hole i aligned with the direction of the airflow when inerting the probe through the wall of the duct. (9) Switch on the main power. (10) Check that the LED temperature meter and watt-meter are illuminated. Check that the temperature meter indicate ambient temperature. (11) Increae the heater power in the exchanger by rotating the power control knob clockwie. he power upplied to the exchanger hould be hown in waft on the meter. (12) Switch on the fan and increae the peed by rotating the fan peed control knob clockwie. (13) Oberve that the air velocity i indicated on the indicator. (14) Check that the plate temperature (k) increae. (15) Set the heater power control and fan peed control knob to minimum. (16) Now you are ready for the following experiment Shutdown Procedure (1) Switch off the heater and turn the power control knob fully anti-clockwie. (2) Set the fan peed control knob to maximum to cool down the hot plate heat exchanger. (3) urn off the main power upply after plate heat exchanger ha cooled down to room temperature.

5 5 3. EXPERIMEN: Free Convection Heat ranfer Procedure: (1) Remove the fan aembly from the top of the duct. (2) Place the pinned heat ink into the tet duct. (3) Record the ambient air temperature ( ). (4) Set the heater power control to Watt (clockwie). (5) Allow ufficient time to achieve teady tate condition before noting the heated plate temperature ( ) into the table below. (6) Repeat thi procedure at Watt. (7) Plot a graph of power againt temperature ( ) (8) Replace the finned heat ink into the tet duct, and then repeat procedure (3) (7) Ambient air temperature = Heat Sink ype Power Input Plate emperature emperature Difference Q [W] 1. Pinned heat ink 2. Finned heat ink

6 6 4. EXPERIMEN: Forced Convection Heat ranfer Procedure: (1) Place the fan aembly on to the top of the duct. (2) Place the pinned heat ink into the duct. (3) Note the ambient air temperature ( ). (4) Set the heater power control to Watt (clockwie). Allow ufficient time to achieve teady tate condition before noting the heated plate temperature ( ). (5) Set the fan peed control to give a reading of V 1 on the thermal anemometer, allow ufficient time to achieve teady tate condition. Record heated plate temperature. (6) Repeat thi procedure at velocity level V 2, V 3 and V 4 (7) Repeat thi procedure at Watt. (8) Plot a graph of air velocity againt temperature. ( ) (9) Replace the finned heat ink into the tet duct, and then repeat procedure (3) (8)

7 Pinned heat ink Ambient air temperature = Power Input Air Velocity Level Plate emperature emperature Difference Q [W] [m/] 4.2. Finned heat ink Ambient air temperature = Power Input Air Velocity Level Plate emperature emperature Difference Q [W] [m/] 5. Reult and dicuion Report the previou reult, ample calculation, graph and the dicuion in a technical report format.