NATURAL/FREE CONVECTION
- 1. NATURAL CONVECTION Y U G M O D I 1 7 1 0 3 0 0 1 1 0 3 8 Student of IIT-RAM, Ahmedabad Guided by Dr. Ajitkumar HEAT AND MASS TRANSFER
- 2. WHAT IS CONVECTION?? • Convection is the transfer of heat by the movement of a fluid (liquid or gas) between of different temperature. Warm air is less dense than cold air, and so convection currents can form in the presence of a temperature gradient • Newton’s law of Cooling: Q = h·A·(Ts- T) • Type of convection. 1. Free/natural convection 2. Force convection
- 3. WHAT IS FREE/NATURAL CONVECTION? • Natural convection is a mechanism, or type of heat transport, in which fluid motion is generated only by density differences in the fluid occurring due to temperature gradients, not by any external source (like a pump, fan, suction device,
- 4. DAILY LIFE EXAMPLE OF NATURAL CONVECTION Boiling water - The heat passes from the burner into the pot, heating the water at the bottom. ... Radiator - Puts warm air out at the top and draws in cooler air at the bottom. Steaming cup of hot tea - The steam is showing heat being transfered into the air. Ice melting - Heat moves to the ice from the air.
- 5. Hot air balloon - A heater inside the balloon heats the air and so the air moves upward. This causes the balloon to rise because the hot air gets trapped inside. Frozen material thawing - Frozen food thaws more quickly under cold running water that if it is placed in water. The action of the running water transfers heat into the food faster.
- 6. NATURAL CONVECTION (THEORY) • Here, The hot egg is exposed to cooler air. • The motion that results from the continual replacement of the heated air in the vicinity of the egg by the cooler air nearby is called a natural convection current. • the heat transfer that is enhanced as a result of this natural convection current is called natural convection heat transfer.
- 7. BUOYANCY FORCE • The upward force exerted by a fluid on a body completely or partially immersed in it is called the buoyancy force. The magnitude of the buoyancy force is equal to the weight of the fluid displaced by the body.
- 8. VELOCITY & TEMPERATURE PROFILE(THEORY) • Consider a vertical hot flat plate immersed in a quiescent fluid body • We assume the natural convection flow to be steady, laminar, and two-dimensional, and the fluid to be Newtonian with constant properties, including density • The velocity and temperature profiles for natural convection over a vertical hot plate are also shown in Figure
- 9. THE GRASHOF NUMBER • The dimensionless parameter in the brackets represents the natural convection effects, and is called the Grashof number GrL, • Grashof number shows criterion of fluid flow, whether it is laminar or turbulent in natural convection. • Grashof number is observed to be about 10^9. Therefore, the flow regime on a vertical plate becomes turbulent at Grashof numbers greater than 10^9.
- 10. APPARATUS Duct assembly Duct traverse probe(T3) Duct inlet probe (T1) Heater Pinned surface Flat surface Finned surface, honeycomb structure Versatile data acquisition system
- 11. E X P E R I M E N T A L S E T- U P
- 12. PROCEDUERE • Fit the chosen heat transfer surface • First we start electric heater and give input of Q =10W. • After starting the heater we wait for 30-40min to attain the steady state • Record the inlet (ambient temp- T1) at initial of duct. • Record the outlet temperature T3,by increasing step of 10mm from 0 to 80mm. • Record the surface temperature of heat transfer plate T2 • Measure the temperature along the fin and pin using magnetic plate. • Repeat the experiment with other two heat transfer surfaces
- 13. FORMULAS
- 14. OBSERVATION TABLE Distance (mm) Flat plate Finned Pinned 0 36.7 37.7 36.2 10 38.7 38.1 36.4 20 37.9 37.9 34.9 30 36.9 37.6 34.7 40 35.2 36.7 34 50 34.3 35.9 33.6 60 33.3 34.5 33.2 70 31.9 33.9 33 80 31.5 32.9 32.4 Finned plate Pinned plate Flat plate Power 10W 10W 10W T1 32.9 31.3 32.2 Ts 68.3 50.7 54.6 To 34.49 34.128 37.7
- 16. G R A P H S & A N A LY S I S
- 17. CONCLUSION Finned plate Pinned plate Flat plate h (W/m^2K) 5.33 21.26 25.88 • In this experiment the value of h(heat transfer coefficient)of flat flat plate is high,but in real life condition or in different experimental processer it may vary.
- 18. THANK YOU