Date :22nd March 2012

CONTENTS

INTRODUCTION3

AIMS & OBJECTIVES3

Objectives3

To investigate Free Convection and Radiation3

Theory3

EXPERIMENT3

Apparatus Used3

Procedure4

RESULTS, CALCULATIONS, OBSERVATIONS & CONCLUSIONS5

Observations During Tests5

Table 15

Table 25

Calculations6

Calculating Power (Watts)6

Calculating Heat Transfer Emissivity (Ɛ)6

Emisssivity of a black body6

Calculating Q rad6

Calculating Q rad6

Calculating Q conv7

Equation for Free Convection7

Percentage values calculation7

Absolute Pressure calculation7

Graph of Pressure Against Temp Difference8

Conclusions8

Conclusion11

Typical Examples of Heat Transfer12

References13

List of Figures, Tables & Graphs14

Heat Transfer Laboratory Sheet I14

Heat Transfer – Free Convection and Radiation Laboratory

INTRODUCTION

The purpose of this lab is to understand natural and forced convection on a cylinder by measuring surface and ambient temperatures and relating the data to convection heat transfer equations.

AIMS & OBJECTIVES

Objectives

To investigate Free Convection and Radiation

1. Determine the emissivity (Ɛ) of an element experimentally. 2. Determine the Heat transfer coefficients by free convection

Theory

Natural Convection: Heat transfer through circulation of fluid due solely to gravity

Forced Convection: Heat transfer through circulation of fluid due to forced fluid movement (fan, pump, etc.)

Radiation: Heat transferred by surface photon emission, typically only significant at T>>Room Temp.

EXPERIMENT

Apparatus Used

Figures 1 below shows the vacuum pump vessel and measuring equipment used

The apparatus consisted of a heated element which was suspended inside a [pressure vessel. The air pressure in the vessel was varied by the use of either a bleed valve or a 240v vacuum pump. The heat input to the e element was varied by up to 10W, the max working temp was not to exceed 200°C and maintained at that temperature or less throughout the experiment.

The heat, power Input, the element, vessel temperatures and the air pressure inside the vessel was determined by the instruments provided for the experiment

Procedure

1) Using the wall mounted barometer the atmospheric pressure was 1018 mB The gauge gives a reading of gauge pressure (diff between the pressure inside the vessel and pressure outside the vessel)

Absolute pressure (P) = pressure gauge reading + atmospheric pressure (mB)

2) Pressure reduced to 2mB and input voltage set to 8.21 volts. 3) Observations and readings taken after 15 mins to allow system to stabilise and readings tabulated. 4) Item 3 repeated with Vacuum pressure reduced by 12, 60, 200, 500 and then finally with the bleed valve fully open tabulated as before. 5) Bleed valve was then fully opened to allow the pressure inside the vessel to meet atmospheric pressure and readings tabulated.

RESULTS, CALCULATIONS, OBSERVATIONS & CONCLUSIONS

Observations During Tests

The initial observations were of the temperature, vacuum pressure and vessel pressures in relation to the inside diameter of the vessel and element assembly.

The Temp Diff verses Abs pressure graph below (Graph 1) shows the temp difference at zero free convection given by the equation for a straight line Y=MX+C

Surface area of the vessel was given as 3070mm², Element Length was given as 152mm and 6.35mm respectively.

The following Tables detail what is actually occurring to temperature and heat transfer inside the vessel. The table below shows the results from the tests carried out, using pressure gauge readings -1015 (mB), -1002(mB), -957 (mB), -815(mB),...