Heat Transfer in Forced Convection Lab Manual for Mechanical Engineering Students, Summaries of Heat and Mass Transfer

Download Heat Transfer in Forced Convection Lab Manual for Mechanical Engineering Students and more Summaries Heat and Mass Transfer in PDF only on Docsity! SNJB's Late Sau Kantabai Bhavarlalji Jain College of Engineering, Chandwad 423101 Department of Mechanical Engineering Heat Transfer Lab Manual Mechanical Engineering 30 Name of Student: Class: SE/ T.E. /BE Div. Subject: Heat Transfer (302043) Academic Year: 2019-20 Date of Completion : __________________ Date of Submission: __________________ Sr. No. Criteria Possible Marks Marks Obtained 1 Team Work (15) 6, 9, 12, 15 2 Understanding experiment and its components (25) 10, 15, 20, 25 3 Diagram(s)/Calculations (15) 6, 9, 12, 15 4 Interpretation of Result and Conclusions (20) 8,12,16,20 5 Q/A (10) 4,6,8,10 6 Timely Submission (15) 6, 9, 12, 15 Total Date: Name & Signature of Teacher Experiment 5 Title. Heat Transfer in Forced Convection 5.1 Aim. To determine the heat transfer coefficient in forced convection. 5.2 Prior Knowledge: Phenomenon of heat convection, Newton’s law of cooling and concept of boundary layer, and average heat transfer coefficient.. 5.3 Practical Outcomes. After experiment performance students must be able to PrO 1. Calculate local and average value of heat transfer coefficient over the entire height of heating rod experimentally. PrO 2. Calculation of average value of heat transfer coefficient using standard empirical relations. PrO 3. Nature of variation of heat transfer coefficient along length of heating rod. SNJB's Late Sau Kantabai Bhavarlalji Jain College of Engineering, Chandwad 423101 Department of Mechanical Engineering Heat Transfer Lab Manual Mechanical Engineering 31 5.4 Introduction The heat transfer in forced convection occurs mechanically and depends on the motion of the fluid. When a fluid motion is caused by some external means, such as a pump or blower, then the convection is called forced convection. In forced convection, the velocity gradients are more effective than the density gradient. Forced convection heat transfer occurs in heat exchangers such as automobile radiators, condensers, and coolers. 5.5EXPERIMENTAL ANALYSIS 5.5.1 Experimental Setup The experimental setup consists of a blower unit fitted with a test pipe as shown in Figure 5.1. Four band-type nichrome heaters surround the test section of the pipe wall. A portion of the heat supplied is conducted through the pipe wall of the test section and then to the flowing air by forced convection. Fig.5.2 Experimental setup for forced convection Three thermocouples (T2 to T4) are attached to the test section. Thermocouples T1 and T5 record the temperature of the incoming and outgoing air. Thermocouple T6 measures the temperature of the atmospheric air. The test pipe is connected with an orifice to measure the flow rate of air through the pipe. A level valve is fitted in the delivery pipe in order to regulate SNJB's Late Sau Kantabai Bhavarlalji Jain College of Engineering, Chandwad 423101 Department of Mechanical Engineering Heat Transfer Lab Manual Mechanical Engineering 34 The properties of air can be taken at mean film temperature Tf fromTableA.4. Tf = 2 s T T   5.6 Specifications 1. Pipe outer diameter d= 38 mm 2. Pipe inner diameter d= 33 mm 3. Length of testsection L= 500 mm 4. Orifice diameter do =22mm 5. Size of the duct = 200 mm × 200 mm × 700 mm 6. Multichannel Digital Temperature Indicator 0-300C using Chromel/Alomel thermocouples. 7. Blower to force the air through pipe. 8. Water manometer to measure air pressure difference. 9. Dimmer-stat 2 Amp. 240 Volts 10. Controls -a) Voltmeter- 0 to 200 volts. b) Ammeter- 0 to 2 Ampere 5.7 Procedure 1. Switch on the heater and adjust the voltage input to the heater through the dimmer- stat. 2. Adjust the air flow rate with the help of the valve fitted in the delivery valve. 3. After reaching the steady state, take the readings of all thermocouples, voltmeter, and ammeter. 4. Measure the manometer deflection in the U tube with the help of the attached scale. 5. Repeat the procedure two to three times with a different power input to the heater and air flow rate. 5.8 Observation Table Sr. No. Heater input Manomete r reading, Hw Thermocouple readings, °C V I 1 2 3 4 5 6 SNJB's Late Sau Kantabai Bhavarlalji Jain College of Engineering, Chandwad 423101 Department of Mechanical Engineering Heat Transfer Lab Manual Mechanical Engineering 35 5.9. Calculations Calculation for reading No.is on separate sheet attached. 5.10Result The heat transfer coefficient in forced convection=_________W/m 2 .K. Plot of the thermocouple location versus the heat transfer coefficient is shown in Fig. 5.3. Fig. 5.3 Variation of the heat transfercoefficient in forced convection 5.11Conclusions The heat transfer rate in forced convection is more than natural convection. As the flow rate of the fluid increases, the value of the heat transfer coefficient also increases. Further, due to the decrease in fluid density with the temperature rise, the turbulence increases in the flow, which causes an increase in the heat transfer coefficient. QUESTIONS: 1. What is forced convection? 2. Define following i. Reynolds Number. ii. Nusselt Number. iii. Prandtl Number. iv. Peclet Number. 3. What is physical significance of Reynolds number? 4. What is meant by thermal boundary layer? SNJB's Late Sau Kantabai Bhavarlalji Jain College of Engineering, Chandwad 423101 Department of Mechanical Engineering Heat Transfer Lab Manual Mechanical Engineering 36 5. Calculate the approximate Reynolds numbers and state if the flow is laminar or turbulent for the following : (i) A 10 m long yatch sailing at 13 km/h in sea water, = 1000 kg/m 3 and = 1.3 × 10 –3 kg/ms. (ii) A compressor disc of radius 0.3 m rotating at 15000 r.p.m. in air at 5 bar and 400°C and dynamic viscosity is given by (iii) 0.05 kg/s of CO2gas at 400 K flowing in a 20 mm dia. pipe and ANSWERS: Attached separate sheet. Multiple Choice Questions 1. In force convection, the Nusselt number (Nu) depends on a. Pr and Re b. Gr and Re c. Gr and Pr d. none of the above 2. What is the correct formula for the rate of heat transfer by convection form a surface of area A at temperature T to the surrounding fluid at temperature T0? a. qc = h A (T – T0)4 b. qc = h A (T4 – T0 4) c. qc = h A (T – T0) d. none of the above 3.What is the mode of heat transfer from the hot surface to the adjacent layer of fluid which surrounds the surface? a. conduction mode of heat transfer b. radiation mode of heat transfer c. convection mode of heat transfer d. none of the above 4. Mark the system where heat transfer is given by forced convection a) Chilling effect of cold wind on warm body b) Fluid passing through the tubes of a condenser and other heat exchange equipment c) Heat flow from a hot pavement to surrounding atmosphere d) Heat exchange on the outside of cold and warm pipes 5. Nusselt number is given by a) h L/k b) 2 h L/k c) 3 h L/k d) 4 h L/k