Boiling heat transfer

1. Difference B/W Evaporation & Boiling Evaporation -occurs at liquid-vapor interface when vapor pressure is less than saturation pressure of the liquid at a given temperature e.g. evaporation from water at 20 C. - No bubble formation or bubble movement involved

2. Cont….. Boiling - occurs at solid-liquid interface when liquid is brought into contact with a surface at temp Ts, above saturation temp Tsat of the liquid - It involves rapid bubble formation at solid-surface that detach from surface and rise to top surface

3. Boiling occurs when a liquid is brought into contact with a surface at a temperature above the saturation temp of the liquid

4. Laws Applicable to Boiling Newton's law of cooling q boiling =h(T s - T sat )=h ∆ T excess ∆ T excess =T s - T sat = Excess temperature T s =temperature of heating surface T sat = saturation temp of liquid

5. Classification of Boiling 1) Based on Bulk Fluid Motion a. Pool Boiling b. Flow Boiling 2) Based on Bulk liquid temperature a. Sub-cooled Boiling b. Saturated Boiling

6. Pool Boiling Boiling in absence of bulk fluid flow Fluid body is stationery Any possible fluid motion will be due to natural convection currents E.g boiling of water in a pan on stove

7. Flow Boiling/Forced convection boiling Boiling in presence of bulk fluid flow Fluid is forced to flow in a heated pipe or over a surface by pump etc Convection effects will be present

8. Sub-cooled/Local Boiling Boiling is sub-cooled if temperature of main body of fluid is below the saturation temp T sat (i.e. bulk of liquid is sub-cooled) It occurs at early stages of boiling Bubbles formation and disappearance near hot surface Bubbles disappear as they transfer heat to surrounding sub-cooled liquid Boiling is confined to locality of hot surface so also called local boiling

9. Cont…… Bubbles serve as energy movers and transfer heat to fluid by condensing

10. Saturated/Bulk Boiling Boiling is saturated if temperature of main body of fluid is equal to the saturation temp T sat (i.e. bulk of liquid is saturated) It occurs when entire liquid body reaches saturation temperature Bubbles rise to the top

11. Boiling Curve for Pool Boiling Four regimes/phases for pool boiling with change in excess temperature are Natural Convection Boiling Nucleate Boiling Transition Boiling Film Boiling

12. Natural Convection Boiling Fluid motion in this regime is by natural convection currents Heat transfer from heating surface to fluid is by natural convection Liquid is slightly superheated

13. Nucleate Boiling Bubbles form at nucleation sites (rough surface) Bubbles form, travel & collapse in liquid Vacated space near heated surface is filled by liquid Increased stirring & agitation Increased h and more heat flux Further rise in temp cause the formation of more bubbles and these move to free surface and break up and release vapor

14. Cont…. At large values of ∆ Texcess a larger fraction of heater surface is covered with bubbles and make difficult for liquid to reach heating surface so max heat flux is reached called as Critical/Maximum heat flux point c. Most desirable in industry because with small ∆ Texcess , high heat transfer rates occur

15. Transition Boiling/Unstable regime At this phase larger fraction of heating surface is covered with bubbles, so heat flux decrease vapor film reduces heat transfer b/c its k value is less than liquid Partial nucleate and film boiling occur Usually this regime is avoided in industry

16. Film Boiling Heating surface is completely covered with stable film, heat flux is minimum By increasing temp further, heat transfer occur through vapor film by radiation which is significant at high temperature

17. Boiling Regimes during mecthanol boiling on steam heated copper tube Nucleate Boiling Transition Boiling Film Boiling

18. Boiling Curve

19. Enhancement of heat transfer in pool boiling In nucleate boiling rate of heat transfer depends on active nucleation sites Increasing nucleation sites will increase heat transfer Surface roughness and dirt increase heat transfer Heat flux in nucleate boiling can be increased by a factor of 10 Coat surface with thin layer/ Thermoexcell -E

20. Cont….. Mechanical agitation and surface vibration also increase heat transfer

21. Flow Boiling In pool boiling vapor bubbles rise due to buoyancy forces, but in flow boiling, fluid is forced to move by external source such as pump as it undergoes a phase change process Combined effects of pool boiling and convection

22. Flow Boiling Types A) External Flow Boiling If fluid is forced to move over a heated surface (on external side) B) Internal Flow Boiling If fluid is forced to move inside a heated surface/tube (on internal side)

23. External Flow Boiling It is similar with pool boiling, but added fluid motion increases the nucleate boiling heat flux and critical boiling heat flux Velosity ∞ h ∞ q max

24. Internal Flow Boiling It is complicated as there is no space for vapor for escape, so vapor and liquid flow together, hence two-phase flow occurs. Different floe regimes are present depending on relative amounts of vapor and liquid present

25. Flow regimes in Internal Flow Boiling FC= Total vapors MF=liquid drops suspended in vapor TF= complete dry spot form on tubes inner side AF=core of flow consist of vapors and liquid flow in annular space b/w vapor and tube SF=Bubbles grow and coalesce into slugs of vapor BF= when bubbles appear in liquid FC=Total liquid

26. Flow regimes in Internal Flow Boiling

27. Applications of Boiling -House hold refrigerator( refrigrant boiling) -steam power plants boilers -chemical industries boilers -cooling of nuclear reactor by coolant boiling - industrial kettles electronic component cooling by boiling of liquid in which these are placed Regenerative cooling of rocket motors

28. Condensation Heat Transfer Condensation occurs when temperature of a vapor is reduced below its saturation temperature. Vapor is contacted by a solid surface at a temp well below vapor saturation temp Condensation Types A) Film Condensation B) Drop-wise Condensation

29. Film Condensation Condensate wet the surface and form a liquid film on the surface which fall due to gravity Thickness of film increases as it moves down due to more condensation

30. Film Condensation

31. Drop-wise Condensation Condensed vapors form droplets on surface instead of film, surface is covered by drops Droplets slide down the surface, when they reach a certain size

32. Drop-wise Condensation

33. Heat Exchanger LMTD Definition Log mean temp difference is defined as that temperature difference which, if constant, would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference Industrial Temp Control of Heat Exchangers E-2163 control