Engineering guide
The MESK module provides the thermophysical properties of a coolant: density, specific heat capacity, dynamic viscosity, thermal conductivity and the Prandtl number derived from them, in each case taking the phase state into account. These quantities are the input data for practically every thermal calculation – without consistent fluid properties, neither heat transfer coefficients nor pressure drops can be determined reliably.
In practice, the module is used as a fluid-property building block within larger calculations, for example in the design of cooling circuits, cooling jackets or heat exchangers: anyone who wants to calculate the heat transfer of a coolant needs the Prandtl number and thermal conductivity for the Nusselt correlation, plus density and viscosity for the Reynolds number and the pressure drop. The properties are evaluated at the relevant operating state, so that the temperature dependence of the fluid properties correctly enters the downstream calculations.
Scope
- MESK
Typical workflow
Define coolant and phase state
First, the phase state of the coolant is determined, because property correlations are always valid for one phase only; liquid and vapour/gas differ in density and viscosity by orders of magnitude.
Evaluate properties at the operating state
For the specified state, density, specific heat capacity, dynamic viscosity and thermal conductivity of the coolant are calculated from the stored correlations.
Form derived characteristic numbers
From viscosity, heat capacity and thermal conductivity, the Prandtl number Pr = η·cp/λ is formed – the key fluid-property number for all heat transfer correlations.
Pass the values to the main calculation
The fluid properties feed into downstream modules, for example to calculate the Reynolds and Nusselt numbers, the heat transfer coefficient and the pressure drop of the cooling circuit.
Frequently asked questions
At which temperature should the properties be evaluated?
It is common practice to evaluate at the mean fluid temperature of the section under consideration. For large temperature differences between wall and fluid, many Nusselt correlations additionally require a correction factor using the viscosity or Prandtl number at wall temperature – in that case, the properties may need to be evaluated twice.
Why is the Prandtl number so important for coolant selection?
The Prandtl number describes the ratio of momentum to heat transport. Water (Pr ≈ 2–7) and water-based mixtures transfer heat far better at the same flow conditions than highly viscous oils (Pr up into the thousands); for gases, Pr is around 0.7. Via the Nusselt correlation, Pr directly enters the achievable heat transfer coefficient.
What should be considered for coolant mixtures such as water-glycol?
Additives change all properties at once: glycol, for example, lowers the thermal conductivity and heat capacity and increases the viscosity considerably, especially at low temperatures. An antifreeze mixture therefore needs a higher circulation rate for the same cooling duty and delivers poorer heat transfer than pure water – the properties must be evaluated for the actual concentration.
Technical content source: LV-Solved-Problems-Content 628d313247e7