[PPT] - Effect of Atmospheric Pressure on Wet Bulb Depression Raymond M. PowerPoint Presentation

SLIDE 1

Effect of Atmospheric Pressure on Wet Bulb Depression

Raymond M. Wheeler, Michael A. Stasiak, Jamie Lawson, Cara Ann P. Wehkamp, and Michael A. Dixon

NASA Biological Sciences Office Kennedy Space Center, Florida, USA Department of Environmental Biology University of Guelph Guelph, Ontario, Canada

SLIDE 2

Reduced Pressures for Space Missions?

Reduced gas leakage and hence reduced

resupply costs

Reduced structural mass
Increased potential for finding transparent

materials for space “greenhouses”

Rapid egress for EVAs (spacewalks) without

prolonged prebreathing and acclimation

How do environmental sensors perform at

reduced pressures?

SLIDE 3

Effect of Pressure on Saturation Vapour Pressure

(Rygalov et al., 2004, NASA Ken Space Center , FL)

1 2 3 4 5

Saturation Pressure (kPa)

20 40 60 80 100 120 15°C 25°C 30°C

Total Pressure (kPa)

Steam table values for es: 30ºC = 4.24 kPa; 25ºC = 3.17 kPa; 15ºC = 1.70 kPa (Kennan, Keyes, et al., 1978)

SLIDE 4

Different Equations for Calculating Saturation Water Vapour Pressures

Goff-Gratch (1946) / and Smithsonian Tables (1984) Log10 pw = -7.90298 (373.16/T-1) + 5.02808 Log10(373.16/T)

1.3816 10-7 (1011.344 (1-T/373.16) -1)

+ 8.1328 10-3 (10-3.49149 (373.16/T-1) -1) + Log10(1013.246) with T in [K] and pw in [hPa] Hyland and Wexler (1983) Log pw = -0.58002206 104 / T + 0.13914993 101

0.48640239 10-1 T

+ 0.41764768 10-4 T2

0.14452093 10-7 T3

+ 0.65459673 101 Log(T) with T in [K] and pw in [Pa] Magnus Teten (Murray, 1967) Log10 pw = 7.5 t / (t+237.3) + 0.7858 with t in [°C] and pw in [hPa] Buck (1981, 1996) pw = 6.1121 e(18.678 - t / 234.5) t / (257.14 + t) [1996] pw = 6.1121 e17.502 t / (240.97 + t) [1981] with t in [°C] and pw in [hPa] Sonntag (1994) Log pw = -6096.9385 / T + 16.635794

2.711193 10-2 * T

+ 1.673952 10-5 * T2 + 2.433502 * Log(T) with T in [K] and pw in [hPa]

SLIDE 5

→ All of these equations are related to saturation pressure of pure water vapour, but water vapour in air does not behave as a completely ideal gas and a corrections are required.

17.502 T e’s =( f ) 6.1121 exp 240.97 + T

Buck (1981) Equation:

Where f = the “enhancement factor” for calculating vapor pressure of moist air instead of pure water vapor. Buck (1981) J. Appl. Meteorol. 20:1527-1532.

SLIDE 6

Effect of temperature and pressure on enhancement factor for correcting moist air properties to that of pure water vapor. From: D.C. Shallcross. 2005. Intl. J. Heat and Mass Transfer 48:1785-1796.

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SLIDE 7

Effects of Pressure on Evaporation Rates

(Rygalov et al., 2004)

2 4 6 8 10 12 14 16

25 50 75 100

Pressure (kPa)

Evaporation Rate (L m-2 d-1)

95% 65% 50%

Relative Humidity → Related to increased gas diffusion rates at reduced pressures

SLIDE 8

Diffusion Coefficient (Dv) of Water Vapour at 25ºC

100 200 300 400 500 600 20 40 60 80 100 120

Pressure (kPa) Diffusion Coefficient Dv (mm-2 s-1)

Fick's Law for Molecular Diffusion: JB = -C Dv [d(CB/C)/dy] where Dv is the mass diffusivity

r the “diffusion coefficient”

and Dv = (0.926/P)[T2.5 /(T+245)]

SLIDE 9

If evaporation rates increase at reduced pressures….. then wet-bulb (WB) depression should also increase.

SLIDE 10

Psychrometric Equation

using Wet Bulb Temperature

es’ = e + γ (Tdb – Twb )

γ = the psychrometric constant

where γ = p A

with p = pressure and A ≈ 6.53 x 10-4 K-1 for average size thermometers and aspiration rate of 4 m s-1

But e

s’ is saturation vapour pressure at the wet bulb temperature !

Thus this equation can’t be used the to solve directly for Twb .

SLIDE 11

Psychometric chart for water vapor in Martian atmosphere brought to 50 kPa pressure. From: D.C. Shallcross. 2005. Intl. J. Heat and Mass Transfer 48:1785-1796.

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SLIDE 12

Themodynamic Wet Bulb Temperature vs. Pressure

(at 25ºC Dry Bulb in Air and Martian Atmosphere)

0.0 5.0 10.0 15.0 20.0 25.0 50 100 150 200

Pressure (kPa) Temperature (ºC)

Open Symbols -- Air ( 78% N2 21% O2 ) (MW = 28.96) (from Shallcross 1997; 2005) Solid Symbols -- Martian Atmosphere (95% CO

2 )

( MW = 43.23)

70% RH 50% RH 30% RH

SLIDE 13

Psychrometric Chart for Pressure using “Composite” Thermodynamic Properties

H.-S. Ren. 2004. Construction of a generalized psychrometric chart for different pressures. J. Mech. Eng. Ed. 32(3):212-222.

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SLIDE 14

Composite Psychrometric Chart Nomograph

H.-S. Ren. 2004. Construction of a generalized psychrometric chart for different pressures. J. Mech. Eng. Ed. 32(3):212-222. p,

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SLIDE 15

Thermodynamic Wet Bulb Temperature vs. Pressure

(at Dry Bulb of 25ºC) 10 15 20 25 40 50 60 70 80 90 100 110

Pressure (kPa) Temperature (ºC)

Data estimated from "Composite“ Psychrometric Charts Ren (2004)

Relative Humidity 90% 70% 50% 30%

SLIDE 16

Our objective was to directly measure wet bulb depression at different pressures and compare our results published psychrometric models for pressure effects.

SLIDE 17

Experimental Approach

Measure wet bulb temperatures five different

pressures and three different relative humidities:

–Pressures: 10, 20, 50, 80, and 100 kPa –Relative Humidities: 30, 50, and 70% Each combination allowed to equilibrate for at least 90 minutes, then a 30-min segment of data was averaged for WB, DB, Dew Point, Chamber Air Temperature, Chamber RH, and Water Temperature

SLIDE 18

Hypobaric Test Chamber University of Guelph, CESF

SLIDE 19

Environmental Monitoring and Control:

Wet Bulb / Dry Bulb

– Enercorp Model HT-WD-A Psychrometer

Two matched platinum RTD temperature probes
Constant aspiration -- 3 m s-1
Humidity Control

– Honeywell Model HIH-3602-A Capacitance Sensors (2)

Temperature Control

–Argus TN 21 Thermisters (2)

Dew Point Measurements

– General Eastern Model 1100DP (1)

Humidity Calibration / Comparison (at 100 kPa)

–Vaisala HMP42 Handheld RH/Temp Probe

Pressure Monitoring / Control

– MKS ‘Barotron’ Capacitance Manometer

Water temperature for the psychrometer reservoir

SLIDE 20

Wet / Dry Psychrometer

Enercorp Inst. Ltd. (Model HT-WD-A)

Air Inlet Water Reservoir Exhaust Fan Signal Transmitter RTD Temp Probe Tube with Cotton Wick

SLIDE 21

SLIDE 22

Wet Bulb Measurements versus Atmospheric Pressure

y = 3.002 Ln(x) + 1.349 R

2 = 0.993

y = 1.593 Ln(x) + 10.23 R

2 = 0.976

y = 0.826 Ln(x) + 16.64 R

2 = 0.722

5 10 15 20 25 20 40 60 80 100 120

Pressure (kPa) Temperature (ºC)

Filled symbols represent our Wet Bulb measurements; Open symbols are adiabatic saturation temperatures from Shallcross (1997, 2005) and Ren (2004) (red triangles). 30% RH 50% RH 70% RH

Dry Bulb Temp. = 25°C

SLIDE 23

Conclusions

Our measurements of wet bulb depression at different

pressures matched the modeled adiabatic saturation temps reasonably well.

At a dry bulb temp of 25°C, the normal wet bulb temp for

30% RH and 100 kPa is ~15°C, but this dropped to ~8°C at 10 kPa.

The results suggest that psychrometers need direct

calibration at the target pressures or that pressure corrected charts are required.

For a given vapour pressure deficit, any moist surfaces,

including transpiring plant leaves, will be cooler at lower pressures due to the increased evaporation rates.

SLIDE 24

Thanks to the CESRF Team at University

f Guelph

Mike Stasiak Jamie Lawson

SLIDE 25

Questions ? Welcome to Florida !

SLIDE 26

Wet Bulb Temperature vs. Adiabatic Saturation Temperature Wet Bulb Temperature: The temperature of a sensor covered with pure water that is evaporating freely into the ambient air

stream. Typically taken with a “matched” dry bulb (DB) reading

under constant aspiration (3-5 m s-1) and shielded from

radiation. But WB readings can be affected by the aspiration

rate, mass of the sensor, water temperature, properties of the wick, and water purity. Adiabatic Saturation Temperature (also called Thermodynamic Wet Bulb Temperature): The thermodynamic state resulting from adiabatic saturation, where there is no heat or mass transfer involved, and is independent of the measurement technique.

SLIDE 27

The Psychrometric Chart

SLIDE 28

Averaged* Chart of Dynamic WB Temperature vs. Pressure

y = 3.245 Ln (x) - 0.410 ; R2 = 0.992 y = 1.844 Ln (x) + 9.445 ; R2 = 0.980 y = 0.897 Ln (x) + 16.88 ; R2 = 0.976

0.0 5.0 10.0 15.0 20.0 25.0 50 100 150 200 250

Pressure (kPa) Temperature (ºC)

30% RH 50% RH 70% RH

Data for 12 and 20 kPa for CO2 (95%) atmosphere; 80, 140, and 200 kPa for air; 50 and 100 averaged for CO2 and air (Shallcross, 1997).