How do I use temperature units?

Question:

What temperature units does nTop support?

How do the temperature units work?

How do I type in the units?

When do I use Absolute vs Relative?

Relative Temperature Property

New Overloads

Temperature Point Map

Applies to:

  • nTopology 2.27
  • Thermal Analysis

Answer:

What temperature units are supported?

nTopology supports Fahrenheit and Celsius temperature units, along with the Kelvin units that are already supported.

How do the temperature units work?

The temperature units are separated into Absolute Temperature and Relative Temperature. This is due to temperature units behaving differently than other units. Temperature units have a scale factor and an offset (since the 0 for K, C, and F is different)

    • Other units (ex. Length):
      • Scale Factor: 1 in = 25.4 mm
    • Temperature:
      • Scale Factor and Offset: 100F = 5/9 K/F * 100F +255.372 = 310.928K

Absolute Temperature: Always shown with K, C, or F. Ex. The face of a part is 200C.

    • Can't mix with other units
    • A new type introduced just for Absolute Units: Temperature and Temperature Field. This cannot be mixed with other units. Can be in K, C, or F and it shows up as K, C, or F for units (depending on the temperature units settings)
      • ex. Applied Temperature input in Applied Temperature Load block
      • ex. Ambient temp input in Radiation Boundary Load block

Relative Temperature: Always shown with K or F. Ex. 20 Btu/(hr*ft*dF).

    • Can be mixed with other units (ex. specific heat capacity J/kg*dC)
    • If temperature units are used in a Scalar or Scalar Field they will represent a Relative Temperature
    • 1K = 1dC = 1.8dF
    • In a scalar, your temperature can be entered as K, dC, or dF. The block will show units in K (for K and dC) or dF

How do I type in the Units?

If your temperature units are set to K, you can still type in values that include dF and dC (such as material properties that are taken from tables such as Btu/(h*ft*dF) or J/(kg*dC)).

Kelvin: K. Absolute and Relative temperatures are the same.

Celsius: Absolute temperature is in C. For Relative temperature, you can type in dC, but since dC and K mean, it will be shown as K. 

Fahrenheit: F for Absolute and dF for Relative.

 

When do I use Absolute Temperature and Relative Temperature?

To decide this, you have to know what you are trying to achieve. Adding two Absolute Temperatures isn't supported since it doesn't make sense. However, it is common to add an Absolute Temperature with a Relative Temperature. 

    • Absolute and Relative: a wall is 200C and will increase by 20dC. 200C + 20dC = 220C ((273.15+200)K + 20K = 493.15K which is 220C)
    • Absolute and Absolute: for the same wall, 200C + 20C doesn't give the same result as the relative temperature because you need to convert to K.  (273.15+200)K + (273.15+20)K = 766.3K which is not the same as the result above, therefore adding Absolute and Absolute isn't supported.
    • Relative and Relative: a wall is xC and it will increase by 200dC and increase by another 20dC. 200dC + 20dC = 220dC which works for specific cases (meaning the total temperature increase)

 

Temperature Properties

An Absolute temperature has a relative temperature property (in C, F, or K depending on the unit setting) and it auto converts to it. Ex:
  • (Absolute) 1K is already in Kelvin and has a (Relative) 1K as a property. (K units are the same for Absolute and Relative)
  • (Absolute) 1C is 274.15K in Kelvin and has a (Relative) 274.15K (remember 274.15K or 274.15dC is the same) as a property.
  • (Absolute) 1F is 255.9278K in Kelvin and has a (Relative) 255.9278K (shown as 255.9278K or as 460.67dF) as a property.
Similarly, a relative temperature (Scalar) has an absolute temperature property and it auto converts to it.

 

New Overloads

    • For the Add and Subtract functions, there are two new overloads: “Temperature, Scalar” and “Temperature Field, Scalar Field” to deal with the mixed addition/subtraction of absolute and relative (ex. 200C +/- 20dC).
    • For the Subtract function, there are two new additional overloads: “Temperature, Temperature” and “Temperature Field, TemperatureField” to calculate the difference between two absolute temperatures in order to get the relative difference (200C - 180C = 20dC or 20K).

 

Temperature Point Map Update

    • This block is updated to output an Absolute temperature result
    • Can be used with Field From Point Map to create a Temperature Field

More on this topic:

Keywords:

 field property map question unit temperature temp units Kelvin K C celsius fahrenheit F dF dC absolute relative 
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