1 / 22

HEAT PROCESSES

HEAT PROCESSES. HP6. Heat transfer external flows.

muniya
Download Presentation

HEAT PROCESSES

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. HEAT PROCESSES HP6 Heat transfer external flows Heat transfer at outer flows around sphere, cylinder and pipe bundle (derived asymptotic formula Nu=2 for sphere, paradox of cylinder). Experiment: hot air blown from hair drier to metallic cylinder with thermocouple; air flowrate calculated from temperature differences. Correlations VDI Warmeatlas. Heat exchangers: powerpoint presentation of HE design. Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

  2. HEAT TRANSFER EXTERNAL FLOWS HP6 Wake Nu~Re2/3 Tw Free stream temperature (it was mean calorific temperature at internal flows ) T Thermal boundary layer~1/Re Re=800 Re=200 Example: cross flow - bundle

  3. x HEAT TRANSFER EXTERNAL FLOWS HP6 Parallel flow at plate (thermal boundary layer - Laminar flow)

  4. HEAT TRANSFER EXTERNAL FLOWS HP6 Rear side (wake) Flow around a sphere (Whitaker) Front side Important for heat transfer from droplets… See next slide 0,71Pr380 3,5Re7,6.104 . Cross flow around a cylinder (Sparrow 2004) Important for shell&tube and fin-tube heat exchangers 0,67Pr300 1Re105 . Cross flow around a plate (Sparrow 2004)

  5. D HEAT TRANSFER EXTERNAL FLOWS HP6 Cross flow around a cylinder according to VDI Wärmeatlas is based upon modified definition of characteristic length Laminar flow (theoretical correlation for parallel flow at plate) Turbulent flow Blended correlation

  6. D HEAT TRANSFER tutorial HP6 Compare correlations for the cross flow around a cylinder according to VDI Wärmeatlas and Sparrow Graph calculated for Pr=1. Nu,Re defined by diameter D

  7. Tw T D r Tw T=Tw D r HEAT TRANSFER EXTERNAL FLOWS HP6 Heat transfer from a sphere. Limiting case for Re=0 Heat transfer from a cylinder. Limiting case for Re=0 Infinitely long cylinder is so powerful heat source that it can heat the whole space to its surface temperature!! Cylinder is really something extraordinary. There doesn’t exist for example something similar to a linear relationship between velocity and the drag force on sphere (F=6Ru).

  8. D T T 35.1 0C Example EXTERNAL FLOWS HP6 How to determine heat transfer coefficient experimentally • Procedure: • Record temperature T(t) • Evaluate time constant  • Evaluate  Thermal resistance of body has to be much less than the thermal resistance of thermal boundary layer s is thermal conductivity of solid (not fluid)

  9. HEAT PROCESSES tutorial HP6 Identify heat transfer coefficient (cross flow around cylinder) Pt100 T [C] Cylinder H=0.075, D=0.07 [m] Aluminium cp=910, rho=2800 kg/m3 Df=0.05m Air cp=1000, =1 kg/m3, =0.03 W/m/K 1400C 190C FAN (hot air) OMEGA data logger (thermocouples) T1,T2 , T3 Watt meter Measured 1.3.2011 1200 W

  10. Experiment 1.3.2011 =585 s T0=19.2, T=81 C HEAT PROCESSES tutorial HP6 Example: velocity of air calculated from the enthalpy balance is 5 m/s (Tnozzle=140 0C, mass flowrate of air 0.01 kg/s) Corresponding Reynolds number (kinematic viscosity 2.10-5) is Re=17500 Nusselt number calculated for Pr=0.7 is therefore This is result from the heat transfer correlation More than 2times less is predicted from the time constant Probable explanation of this discrepancy: Velocity of air (5m/s) was calculated at the nozzle of hair dryer. Velocity at the cylinder will be much smaller. As soon as this velocity will be reduced 5-times (1 m/s at cylinder) the heat transfer coefficient will be the same as that predicted from the time constant (76 W/m/K)

  11. recommended for reading HEAT TRANSFER EXTERNAL FLOWS HP6 Ephraim M. Sparrow, John P. Abraham, Jimmy C.K. Tong: Archival correlations for average heat transfer coefficients for non-circular and circular cylinders and for spheres in cross-flow. International Journal of Heat and Mass Transfer 47 (2004) 5285–5296 Equivalent diameter Dh is replaced by width l in Nu and Re definition

  12. HEAT TRANSFER EXTERNAL FLOWS HP6 S. Tiwari, D. Maurya, G. Biswas, V. Eswaran: Heat transfer enhancement in cross-flow heat exchangers using oval tubes and multiple delta winglets. International Journal of Heat and Mass Transfer 46 (2003) 2841–2856 Vortex shedding behind circular pipe increases pressure drop Flow behind an oval tube is almost steady and symmetric Air flow in narrow gap of fin-tube heat exchanger is usually laminar Nu enhancement by winglets

  13. HEAT TRANSFER Bundle of tubes cross flow HP6 Delvaux

  14. HEAT TRANSFER Bundle of tubes cross flow HP6 Zhukauskas (1972) 0,7Pr500n=0 gases n=0,25 liquid Exponent mdepends uponRe (increases from laminar value 0,4 up to 0,84 in fully turbulent flow), Coefficientc1depends uponRe and geometry. The coefficientc2depends upon number of tube rows in the bundle.

  15. HEAT TRANSFER Bundle of tubes HP6 Procedure recommended by VDI Warmeatlas Nussels number for 1 row of tubes calculated from correlation (e.g.Sparrow) for single tube using modified velocity in Re Nusselt number for N-rows of tubes in the direction of flow (Nu increases with number of rows because tubes act as a promotor of turbulence)

  16. HEAT TRANSFER Film flow HP6 Wyeth

  17. HEAT TRANSFER Film flow HP6 Free film of liquid falls down on a vertical heat transfer surface driven by gravity (water coolers, vertical shell&tube heat exchangers, falling film evaporators). Mass flow rate  (kg/s/m) is determined by a liquid distributor at the top of wall (usually a vertical tube).  (=u) determines the flowing film thickness  as follows from the force balances y  Parabolic velocity profile gravity Viscous force at wall Correlation for laminar film Re=/ < 400 Turbulent film

  18. HEAT TRANSFER Film flow HP6 Falling film evaporators – liquid film flows down on inner surface of vertical tubes, heated from outside. Heat transfer coefficient is usually calculated as =/. Nii S.et al: Membrane evaporators. Journal of membrane science, 201 (2002), 149-159 Mass flowrate has to be high enough so that a uniform and stable liquid film covering the whole heat transfer surface of tubes will be formed. Stability and waviness of the film is affected by surface tension . Restriction on minimal flowrate (intensity of scrapping) can be expressed in terms of Weber number (ratio of kinetic and surface energy) Flow Distribution into tubes of evaporator calandria Consequence: the heat transfer coefficient cannot be greater than Very restrictive!!

  19. HEAT TRANSFER wiped film HP6 Scraped surface heat exchangers are applied for processing of highly viscous and fouling sensitive materials. Contact time corresponding to thermal boundary layer development Penetration depth Thickness of boundary layer  N-number of blades, n-rotational frequency This is only an idea, more precise Azoory Boot correlation See also R. De Goede, E.J. De Jong: Heat transfer properties of a scraped-surface heat exchanger in the turbulent flow regime. Chemical Engineering Science, Volume 48, Issue 8, 1993, Pages 1393-1404

  20. HP6

  21. EXAM HP6 Heat transfer external flows

  22. What is important (at least for exam) HP6 sphere derive Nu=2 for Re=0 from equation cylinder cross flow on bundle of tubes (N-rows) heat transfer at falling film

More Related