Objectives

1
Objectives

• Finish with Heat transfer
• Learn about Psychometrics
• Psychometric chart

2
Forced Convection

• External turbulent flow over a flat plate
• Nu hmL/k 0.036 (Pr )0.43 (ReL0.8 9200 ) (µ8
  /µw )0.25
• External turbulent flow (40 lt ReD lt105) around a
  single cylinder
• Nu hmD/k (0.4 ReD0.5 0.06 ReD(2/3) ) (Pr
  )0.4 (µ8 /µw )0.25
• Use with care

3
Natural Convection

• Common regime when buoyancy is dominant
• Dimensionless parameter
• Rayleigh number
• Ratio of diffusive to advective time scales
• Book has empirical relations for
• Vertical flat plates (eqns. 2.55, 2.56)
• Horizontal cylinder (eqns. 2.57, 2.58)
• Spheres (eqns. 2.59)
• Cavities (eqns. 2.60)

4
Phase Change Boiling

• What temperature does water boil under ideal
  conditions?

5
Radiation

• Transfer of energy by electromagnetic radiation
• Does not require matter (only requires that the
  bodies can see each other)
• 100 10,000 nm (mostly IR)

6
Surface Radiation Issues
1) Surface properties are spectral, f(?) Usually
assume integrated properties for two beams
Short-wave and Long-wave radiation 2) Surface
properties are directional, f(?) Usually assume
diffuse
7
Radiation emission
The total energy emitted by a body, regardless
of the wavelengths, is given by

• Temperature always in K ! - absolute
  temperatures
• emissivity of surface e 1 for blackbody
• Stefan-Boltzmann constant
• A - area

8
Short-wave long-wave radiation

• Short-wave solar radiation
• lt3mm
• Glass is transparent
• Does not depend on surface temperature
• Long-wave surface or temperature radiation
• gt3mm
• Glass is not transparent
• Depends on surface temperature

9
Radiation Equations
10
Combining Convection and Radiation

• Both happen simultaneously on a surface
• Slightly different temperatures
• Often can use h hc hr

11
Humidity Ratio, W

• W mw/ma
• Degree of saturation, µ W/Ws
• Humidity ratio is hard to measure, but very
  useful in calculations
• What are units?
• Is W a function of temperature? What about Ws?

Ws humidity ratio at saturation ma mass of
dry air mw mass of water vapor
12
Relative Humidity

• F xw/xw,s Pw/Pws
• Function of T
• Easy to measure and useful in some contexts, but
  often need to know temperature as well

x mole fraction P pressure µ degree of
saturation W humidity ratio
13
Dew-point temperature, td

• Temperature at which condensation will form
• Under appropriate surface conditions
• Vapor is saturated
• F ?
• Ws(P, td) W

14
Wet-bulb temperature, VBT (t)

• Temperature of wet surface or
• Temperature at which water, by evaporating into
  the air, will bring air to saturation
  adiabatically
• superscript is designation that variable is
  evaluated at the wet-bulb temperature
• Note, distinct from that measured by a sling
  psychrometer
• Section 9.5

15
Tables for Moist Air (P 1 atm)

• Tables A.4 in your text
• Ability to get Ws for calculations
• Subscripts
• a dry air, s saturated air

v vaµvas h haµhas s saµsas
16
(No Transcript)
17
Psychrometric Chart

• Need two quantities for a state point
• Can get all other quantities from a state point
• Can do all calculations without a chart
• Often require iteration
• Many digital psychrometric charts available
• Can make your own
• Best source is ASHRAE fundamentals (Chapter 6)
• Also in your text (back cover fold-out)

18
Ref Tao and Janis (2001)
19
Ref Tao and Janis (2001)
20
Ref Tao and Janis (2001)
21
Ref Tao and Janis (2001)
22
Examples

• What is enthalpy of air in the classroom right
  now?
• Condensation on windows when taking a shower
• How cold does it have to be outside for
  condensation to form on windows?
• Assumption is that windows are the same
  temperature as outside air
• 80 F, RH 80

23
(No Transcript)
24
Alternate calculation for W

• PV mRT (IGL)
• What do we know about R ratio?
• P Pw Pa

R gas constant P pressure V volume T
absolute temperature W humidity
ratio Subscripts w is water vapor, a is dry air
25
Calculation of psychometric quantities

• For an ideal gas,
• hda ?cpadT, hw ?cpwdT
• So, hda cp,dat which assumes a reference state
  of 0
• F or 0 C Tables A4
• Note different reference
• hw cpwt hg0
• h cp,dat W(cpwt hg0)
• Or you can use
• h cpt Whg0, cp cp,da Wcpw

cp specific heat h enthalpy T absolute
temperature t temperature W humidity
ratio Subscripts w is water vapor, a is dry
air, g is saturated water vapor
26
Adiabatic mixing

• Governing equation

External heat
27
Sensible heating
28
Dehumidification by Cooling
29
Real Dehumidification Process
30
Mold in a duct
Transport of saturated air
tsurface lt tdp
Condensation
31
Humidification
hw Specific enthalpy of water added to
system hg Specific enthalpy of saturated water
vapor
32
Summary

• Describe psychrometric quantities
• Given any two psychrometric quantities, calculate
  any other quantity
• Use Tables A4 or psychrometric charts to look up
  psychrometric quantities
• Calculate psychrometric quantities at
  non-standard conditions