How to choose... model - pressure based or density based Ansys Fluent ?

 One of the first steps in the preprocessor in Ansys Fluent is selecting a fluid mechanics solver. This is the first decision block that will determine the direction in which we want to define our boundary and initial conditions, which strictly depends on our assumptions.

Pressure or Density based solver in Fluent 

In this short post I would like to give you a general overview of the differences between solvers that are available in Fluent. But remember that if you choose the wrong solver, you probably won't make a big mistake. For some time, the developers of Fluent have extended these two models towards universality. Pressure based or density based will solve your problems in a similar way, even if they are at two different poles.

For general purpose PRESSURE BASED model was developed for low speed incompressible flows below speed of sound. The DENSITY BASED model was generally used in high speed compressible flows.

 For the most cases of heat transfer problems U pick pressure based model. 

Window where we pick solver method in Fluent 

Detailed description of Solvers (Models)

In the pressure-based solver, the governing equations are solved sequentially i.e. Governing equation for the variables such as u, v, w, p, T, k etc. are solved one after another. 

Unlike sequential pressure-based solver, there is another pressure-based solver which is called coupled solver. In coupled pressure-based solver the momentum and continuity equations are solved simulatneously. the governing equations are solved in coupled manner and other scalar equations are solved in segregated manner. The solution convergence rate is greatly enhanced when compared to sequentially pressure model. But in terms of memory, the coupled need more memory requirement than sequential. 

In density-based solver, the energy, momentum, continuity and species transport equations are solved simultaneously. To design both incompressible and compressible fluid flows, the equations are altered.in density-based solver, time plays an important role whether it can be steady state or transient. density-based solver, transient term is incorporated. The time step is represented by courant number in steady state problem. Limitation of maximum time step will occur due to some stability issues in explicit solver i.e. variables are differentiated with respect to time (e.g. Runge Kutta Scheme) but there will be no issues associated with implicit solver i.e. Variables are differentiated with respect to time as well as spatial co-ordinates. The set of equations are solved simultaneously using second order iteration (e.g. Gauss-Seidal). 

Source:

What is the difference between pressure based solver and density based solver in Ansys Fluent? What is the criteria while selecting one of them? - Quora

However today both methods U can use widely in many problems because they have been reformulated.

 Soo If U pick density based model for thermal analysis and low speed gas flow U won't make big mistake - but U can get higher error deviation (imbalances). 

In most cases if U take pressure based model and ideal gas formulation for the fluid domain U need to define gravity. Only this configuration can fully make Ur model pressure dependent specially for thermal analysis where gas flows and ABS pressure have influence (for example  - forced convection, conjugate heat transfer).