Here are some more state properties of fluids in GUNNS:

Specific Enthalpy (h). Standard GUNNS Trick units is joules per kilogram (J/kg). Enthalpy is a measure of energy in the fluid, and specific enthalpy is the energy per unit mass. Enthalpy includes the fluid’s internal energy (more on that later) and the energy required to displace the environment to make room for it at its volume and pressure. In GUNNS, we use enthalpy to describe energy transfer between mixing fluids and heat transfer between the fluid and its environment.
Specific Heat (Cp). Standard GUNNS Trick units is joules per kilogram per Kelvin (J/kg/K). Specific heat is the amount of heat needed to be added a unit mass of fluid to raise its temperature by unit. This differs depending on whether the heat transfer process occurs at constant pressure (Cp) or volume (Cv), but the constant-pressure heat relates to changes in the enthalpy.
Thermal Capacity. The is a mass of fluid’s capacity to store heat. It is mass * Cp with units (J/K). It’s the exact same thing as capacitance in the thermal aspect, applied specifically to fluids in the fluid aspect. Note that mass is not a fluid property, but an amount of fluid. Cp is the only fluid property here. So thermal capacity is an implied property of a fluid object, not of the fluid type.

Both properties are modeled. There are some simplifications involved that you don’t normally need to worry about.

Enthalpy, specific heat and temperature are related. For an ideal fluid, they’re only a function of temperature:

h = Cp·T

For non-ideal fluids the above is not true, and specific enthalpy and heat are also a function of pressure, but GUNNS always assumes an ideal fluid anyway for simplicity. This imposes some significant limitations which we’ll discuss later.

Thus at the same temperature, different fluid types have differing enthalpies (heat content) in the same ratio as their specific heats. When mixing different fluid types at different temperatures, the fluid with the higher specific heat can absorb more energy; and the resulting mixture temperature tends to weight more towards that fluid’s starting temperature.

The takeaways from this are:

• Different fluid types have different specific heats. The higher the specific heat, the more energy it takes to change the fluid’s temperature.
• Fluids with high specific heats (like liquid water) are good coolants because they can move more heat per unit mass and with less change in temperature.
• Different fluids have different specific enthalpies at the same temperature and pressure.
• When different fluid types with different temperatures mix together, the resulting mixed temperature depends on their relative enthalpies.

Previous Page / Next Page