Over the past decades, engineers have focused on a common goal - to reduce emissions by making industrial processes more efficient and by utilizing renewable energy sources. One possibility to reach this goal can be achieved by employing processes with non-ideal fluids, such as fluids at supercritical conditions in refrigeration, heat pump cycles and power cycles.
Most of the flows in industry are turbulent and hence the need to study turbulence in non-ideal fluids arose. Turbulence in non-ideal fluids is extremely challenging since there are many complex effects at play. One such effect is caused by variations in properties, which also occurs in compressible flows, flows with high concentration gradients, or flows in heat exchangers.

This thesis presents a review of the existing theory on semi-local scaling for variable property flows with the aim to take it one step further and apply it to turbulent heat flux modeling. Two types of variable property cases are analysed in this thesis; (1) low-Mach number flows with uniform pseudo-heating sources, (2) high-Mach number flows with non-uniform viscous heating. For the former, a Direct Numerical Simulation (DNS) data-base, already available at TU Delft, has been post-processed with the main goal to investigate if the semi-local theory can also be applied to thermal turbulence and its modeling. For the latter, additional DNS simulations of high-Mach number channel flows have been performed to investigate how the viscous heating and its correlations can be accounted for in the semi-local scaling framework.

Using a 2-equation heat flux model, we find that modeling thermal turbulence in semi-local scales considerably improves the results for low-Mach number flows. However, for high-Mach number flows, additional unknown (closure) terms arise due to fluctuations in the viscous heating source. A model for the source term in the enthalpy variance equation is successfully proposed. In addition, the DNS study of two high-Mach number flows with constant semi-local Reynolds number profiles shed light on the importance of a newly defined parameter (modified Eckert number) and also unveils one of the most important conditions in which semi-local theory can be compromised, e.g. extreme density gradients.

Heat transfer via thermal radiation is a common occurrence in industry; be it in flue gas, boilers, reactors, or in supersonic combustor such as in a scramjet propulsion.
Numerous studies have been done regarding the phenomena interacting with thermal radiation in different types of turbulent flows.

For compressible flows the effect of thermal radiation on the turbulent field via Turbulent Radiation Interaction (TRI) has been research to a lesser degree than for incompressible flows. Therefore, a more in-depth study on the impact of optical thickness in compressible flows should help create a better understanding regarding this.
This study consists of an investigation into the effect of thermal radiation in a non-reacting supersonic channel flow where the optical thickness of the fluid is changed.
The fluid dynamics is simulated using a Direct Numerical Simulation (DNS) code for compressible turbulence and for the thermal radiation a grey-gas Finite Volume Method is used. A fictitious fluid is used with two different Planck numbers (Pl = 0.1, 0.01). Furthermore, for cases with Pl = 0.01 the constant absorption coefficient is varied between κ = 1, 5, 10.

The effects of thermal radiation on the temperature and density fields are discussed. Changing the optical thickness via the absorption coefficient shows a strong change in the behaviour of the fluctuating fields.
Compressibility is shown to be affected by thermal radiations, where a stronger thermal radiation characterized by a high optical thickness and low Pl number show characteristics of an incompressible flow while being supersonic.

A model to determine the fluctuating incident radiation developed for high optical thickness incompressible flows is applied to this study as-is to investigate if the model is suited. It is shown that the assumptions made for the model, especially regarding thermal structures size, do not hold for compressible fluids and a different approach is needed.

Heat transfer in multiphase flows plays an important role in many industrial applications. For instance, particle-based solar receivers utilize the high absorptivity and heat capacity of dispersed phase in a carrier fluid to improve efficiency and heat transfer. This dispersed phase generally consists of a large number of small particles. It is, therefore, difficult to completely resolve such flows considering their finite size. Usually, these particles are so small, that they can be treated as point particles. This considerably reduces the computational effort, while preserving the essential characteristics of the particle-laden flows. In this thesis, the focus is on heat transfer modulation in a particle-laden turbulent channel flow using direct numerical simulations.

In flows with temperature gradients (for example, channel flow between hot and cold wall), particles absorb heat from hotter regions and release heat to colder regions, thereby enhancing heat transfer through particle feedback flux. On the other hand, presence of particles leads to decay in turbulence resulting in lower turbulent heat transfer. The interplay of this two phenomena can either increase or decrease the overall heat transfer based on Stokes number and thermal Stokes number (ratio of thermal response time to characteristic time scale of the flow).

To investigate the heat transfer modulation in particle-laden channel flow, the existing DNS code developed by Boersma [5] has been modified to include particle transport and heat transfer. The point-particle approach with two way coupling is implemented using trilinear interpolation scheme and 3 rd order Runge-Kutta time marching scheme. The implemented code is validated using the results from literature [20] for a flow with no external heating.

With the developed code, cases with no external source term and external source term with different optical thickness of the fluid have been analyzed. In order to focus only on the fluid-particle interaction, the effect of gravity is neglected and the flow is considered to be incompressible. It has been observed from these simulations that particles play an important role in modulation of heat transfer in such flows. Mean temperature profiles, heat flux mechanisms, temperature variance and budgets of temperature variance are studied extensively in order to understand the underlying phenomenon. It is found that the particle feedback heat transfer is the dominating mode in particle-laden flows.