Indirect specification of data

Basis components of a subset of models can be generated indirectly, through specifying more practically available data, including manufacturers' data. These data may be stored as such on separate files. Indirect specification is possible for the following type of component data and corresponding models:

  1. Reciprocating compressors (file extension cmp ):
    Reciprocating compressor and Cooled reciprocating compressor .
  2. One-sided single phase forced convection heat exchange (file extension hx_s ):
    Heat exchanger with buffer and 1D heat source with external heat exchanger .
  3. Two-sided single phase forced convection heat exchange (file extension hx_ss ):
    Standard heat exchanger and Accurate heat exchanger .
  4. Refrigerant vs. secondary fluid heat exchange (file extension hx_rs ):
    Standard evaporator , Accurate evaporator , Standard condenser , Accurate condenser , Superheat heat exchanger , Subcooling heat exchanger and Gas cooler .
  5. Climate data: (file extension: climate )
    Outdoor climate (Note: This indirect specification has so far been disabled due to insufficient model basis. Specify directly!)

Comment on heat transfer correlations:
The heat transfer coefficient and pressure gradient generated for the two-phase domain are based on single-phase correlations, as there are no general and accurate correlations covering this. Therefore for real simulations the user should calculate better data based on measurements, charts or separate calculation programs. The data has no entry for vapour fraction, as the mass flow rate and temperature were selected as the two preferred input quantities. Therefore the values must be averages over the total evaporation or condensation.

If the data are represented as function files (see input data functions), it can be easily included through the function dialogue load feature. Do a function save first and see the structure of the function file (of type fnc ).

The correlations used are found in physical correlations as General forced convection heat transfer , General free convection heat transfer and Correlations for pressure drop .

Comment on climate data:
When doing a design involving the calculation of heat exchange between atmospheric climate and ground, it is possible to use statistical data, like monthly means over a year, to establish rough estimates. If an accurate design is required, however, it is recommended to use historical data for a number of representative years. Such historical data contain observations several times a day of air temperature, wind speed etc. An important aspect of using historical data is that typical covariations of parameters appear. When using statistical values, one disregards such covariance.

Values for global radiation is not always available. If this is the case, estimates are generated from the other parameters. Note however that the accuracy can be poor, especially in polluted regions. CURRENT STATUS: Due to lack of values for estimating diffuse (scattered) solar radiation, the estimated global radiation is only accounting for direct solar radiation.

The structure of the climate type file is a follows:

TextSite - Text describing the site of the climate data.
angLat,      - Site latitude.
angLong,     - Site longitude.
angLongZone, - Time zone longitude.
lHeight      - Site altitude
    cOpt - Option for specification of climate data:
           'T': Temperature.
           'V': Wind velocity.
           'R': Relative humidity.
           'C': Cloud cover.
           'H': Cloud height.
           'P': Air pressure. (Currently not used)
           'F': Precipitation.
           'Q': Global radiation.
    Time, - Time of climate parameter observation (seconds).
    (cOpt = 'T')
        T, - Temperature at Time (Degrees Celsius).
    (cOpt = 'V')
        V, - Wind velocity at Time (m/s).
    (cOpt = 'R')
        R, - Relativ humidity at Time (% or fraction).
    (cOpt = 'C')
        C, - Cloud cover at Time (% or fraction).
    (cOpt = 'H')
        H, - Cloud height at Time (m).
    (cOpt = 'P')
        P, - Air pressure at Time (hPa).
    (cOpt = 'F')
        F, - Rain fall at Time (mm/day).
    (cOpt = 'Q')
        Q  - Global radiation at Time (W/m2)
The values of R and C can be given in % [0,100] or as fractions [0,1]. Values above 1 are considered to be % values, otherwise fractions.

If global radiation Q is not specified, the following correlations are used to calculated estimated values. There has been a literature search for sturdy estimates of the diffuse component of the solar radiation that reach the ground, adding to the global radiation value. So far nothing useful has been found. Therefore the values calculated are the direct solar radiation component of the global radiation. These are close to the real ones for a clear sky.

Estimation of global radiation:

angDay      = 2*Pi*(Day-(31+28+20.5))/365
angDecl     = angTropic*sin(angDay)
angHour     = 2*Pi*(Hour-12)/24 + angLong - angLongZone
sin(angSol) = sin(angLat)*sin(angDecl)+cos(angLat)*cos(angDecl)*cos(angHour)
I0(Day)     = 1353*(1+0.0342*cos(2*Pi(Day-1.5)/365), W/m2
I(angSol,h) = I0(Day)*(a*h + (1-a*h)*exp(-c/sin(angSol)**s))
I,horz      = I*sin(angSol)*(1-rCover)


angSol      - Solar altitude (Solar angle above horizon)
angLat      - Site latitude
angLong     - Site longitude
angLongZone - Time zone longitude
angDecl     - Solar declination (Solar angle above the earth's equator plane)
angDay      - Angle position of earth vs. the sun, 0 at vernal equinox
              (on average 21 March 12:00)
angTropic   - Maximum solar declination (or tropic latitude) = 23.47/180*Pi
Day         - Day of the year = Time/(Day duration) (1.5 = January 2nd 12:00)
Time        - Time after January 1st 0:00
Hour        - Hour of the day (12 = Sun to the south, 12.25 = Quarter past 12)
rCover      - Cloud cover
I(angSol,h) - Direct solar irradiation on a plane normal to the incident direct rays
I0(Day)     - Extraterrestrial irradiation
a           = 0.00014/m
c           = 0.357
s           = 0.678
h           - Altitude above sea level < 2000 m

Ref.: A. B. & M.P Meinel, "Applied Solar Energy, An Introduction", Addison-Wesley, 1976.

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