NATURAL/FREE CONVECTION

natural/free convection presentation by student IITRAM

1. Natural convection Yug modi 171030011038 Student of IIT-RAM, Ahmedabad Guided by Dr. Ajitkumar Parvani HEAT AND MASS TRANSFER

2. What is convection?? Convection  is the transfer of heat by the movement of a fluid (liquid or gas) between areas 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. Free/natural convection Force convection

3. What is free/natural convection? Natural convection is a mechanism, or type of heat transport, in which the 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, etc.)

4. Daily life example of natural convection

5. 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.

6. 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.

7. 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

8. 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.

9. Apparatus

10. Experimental set-up

11. 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

12. Formulas

13. Observation table Distance (mm) Flat plate Finned Pinned 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

14. Graphs & analysis

15. Graphs & analysis

16. 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.

17. Thank you