Contents1. Objective …………………………………… 1 2. Aim …………………………………… 1 3. Introduction …………………………………… 1 4. Theory …………………………………… 1 5. Description …………………………………… 1 6. Utilities Required …………………………………… 2 7. Experimental Procedure …………………………………… 2 8. Specification …………………………………… 2 9. Formulae …………………………………… 3 10. Observation & Calculation …………………………………… 3 11. Nomenclature …………………………………… 3 12. Precautions & Maintenance Instructions …………………………………… 4 13. Troubleshooting …………………………………… 4 14. References ……………………………………. 4 EMISSIVITY MEASUREMENT APPARATUS 1. OBJECTIVE: Study of Radiation heat transfer by black body and test plate. 2. AIM: To find out the emissivity of a test plate. 3. INTRODUCTION: All substances at all temperature emit thermal radiation. Thermal radiation is an electromagnetic wave and does not require any material medium for propagation. All bodies can emit radiation and have also the capacity to absorb all of a part of the radiation coming from the surrounding towards it. 4. THEORY: An idealized black surface is one, which absorbs all the incident radiation with reflectivity and transmissivity equal to zero. The radiant energy per unit time per unit area from the surface of the body is called as the emissive power and is usually denoted by e. The emissivity of the surface is the ratio of the emissive power of the surface to the emissive power of a black surface at the same temperature. If is noted by E. E = E/Eb For black body absorptivity = 1 and by the knowledge of Kirchoff's Law of emissivity of the black body becomes unity. Emissivity being a property of the surface depends on the nature of the surface and temperature. The present experimental set up is designed and fabricated to measure the property of emissivity of the test plate surface at various temperatures. 5. DESCRIPTION: The experimental set up consists of two circular copper plates identical in size and is provided with heating coils sand witches. The plates are mounted on bracket and are kept in an enclosure so as to provide undisturbed natural convection surroundings. The heating input to the heater is varied by separate dimmerstat and is measured by using an ammeter and a voltmeter with the help of double pole double throw switches. The temperature of the plates is measured by Pt-100 sensor. Another Pt-100 sensor is kept in the enclosure to read the ambient temperature of enclosure. Plate 1 is blackened by a thick layer of lampblack to form the idealized black surface where as the plate 2 is the test plate whose emissivity is to be determined. The heater inputs to the two plates are dissipated from the plates by conduction, convection and radiation. The experimental set up is designed in such a way that under steady state conditions the heat dissipation by conduction and convection is same for both the cases. When the surface temperatures are same the difference in the heater input readings is because of the difference in radiation characteristics due to their different emissivities. 6. UTILITIES REQUIRED: Electricity Supply: 1 Phase, 220 V AC, 4 Amps Table for set-up support(optional) 7. EXPERIMENTAL PROCEDURE: 1. Gradually increase the input to the heater to black plate, adjust it to some value, and adjust heater input to test plate slightly less than the black plate. 2. Check the temperature of the two plates with minor intervals and adjust the input of test plate only, by the dimmerstat so that the two plates will be maintained at the same temperature. 3. This will require some trial and error and may take more than one hour of so to obtain the steady state condition. 4. After attaining the steady state, conditions record the temperatures, Voltmeter, and Ammeter reading for both the plates. 5. The same procedure is repeated for various surface temperatures in increasing order. 8. SPECIFICATION: 1. Test plate dia = 160 mm = .02m² 2. Black plate = 160 mm= .02m² 3. Dimmerstat for both plates = 0-2 A, 0-220V. 4. Voltmeter = 0-300V, Ammeter 0-2.5 A 5. RTD Temperature sensor = 3 Nos 6. Heater for test plate and black plate Nichrome strip wound on mica sheet and sand- witched between two mica sheets of 440 Watt. 9. FORMULAE: 1. Heat input to Black Plate, QB = E A (TS4 - TD4) 2. Heat input to Test Plate, QS = E A (TS4 - TD4) 3. Emissivity of specimen to be determined, E W B WS E Eb A * * (TS TD ) 4 4 10. OBSERVATION & CALCULATIONS: DATA: Stefan Boltzmann Constant = 5.67 x 10-8 w/m2 K4 OBSERVATION TABLE: Black plate: Voltage, V Amperage, I Power input, Wb = V x I Black plate temp, TS (oC) Test plate: Voltage, V Amperage, I Wattage, Ws = V*I TS, oC TD, oC The emissivity of the test plate can be calculated at various surface temperatures of the plates. 11. NOMENCLATURE: QB = Heat input to disc coated with lamp black (watt) Wb = wattage supplied to black plate QS = Heat input to test plate (watt) Ws = wattage supplied to test plate = Stefan Boltzmann Constant A = Area of disc (m2) = 0.02m² Ts = Surface temperature of Discs K TD = Ambient temperature of enclosure K E = Emissivity of specimen to be determined. Eb = Emissivity of black body. 12. PRECAUTIONS & MAINTENANCE INSTRUCTIONS: 1. Use the stabilize A.C. Single Phase supply only. 2. Never switch on mains power supply before ensuring that all the ON/OFF switches given on the panel are at OFF position. 3. Increase Voltage to heaters slowly. 4. Keep all the assembly undisturbed. 5. Never run the apparatus at abnormally low/high voltages 6. Operate selector switch of temperature indicator gently. 7. Always keep the apparatus free from dust. 13. TROUBLESHOOTING: 1. If electric panel is not showing the input on the mains light. 2. If D.T.I displays “1” on the screen kindly inform us. 3. If temperature of any sensor is not displays in D.T.I check the connection and rectify that. 4. If the temperature is not shown proper in D.T.I some air gap is there between the surface of the plate and the sensor. Paste that by using heat sink chemical. 5. Voltmeter showing the voltage given to heater but ampere meter does not. Tight the heater socket & switch if ok it means heater burned. 14. REFERENCES: 1. Holman, J.P., “Heat Transfer”, 8th ed., McGraw Hill, NY, 1976. 2. Kern, D.Q., “Process Heat Transfer”, 1st ed., McGraw Hill, NY, 1965. 3. Perry, R.H., Green, D.(editors), “Perry’s Chemical Engineers’ Handbook”, 6th ed., McGraw Hill, NY, 1985. 4. McCabe, W.L., Smith, J.C., Harriott, P., “Unit Operations of Chemical Engineering”, 4th ed., pp. 360-362, McGraw Hill, NY, 1985. 5. Coulson, J.M., Richardson, J.F., “Coulson & Richardson’s Chemical Engineering Vol. - 1”, 5th ed., pp. 388-389, Asian Books ltd., ND, 1996.