Since the Newtonian era, the empirical heat relation that entailed the proportionality of the rate of heat transfer between two bodies to the temperature difference between the bodies along with a heat transfer coefficient had always been used for industrial applications. Owing to the evolution of computers and a recent discovery by a group of former Soviet scientists, a Conjugate Heat Transfer (CHT) model was developed.
CHT allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them. Moreover, this analysis type eliminates the need for the heat transfer coefficient. The typical applications of CHT include simulating heat exchangers, cooling of electronic equipment, and general-purpose heating/cooling systems.
The current project contains an analysis of a Shell and Tube Heat Exchanger. This is comprised of:
Pipe volume (solid): - Essentially, the housing of the system plus the volume of the pipes containing the working fluid.
Inner fluid volume: This is the fluid flowing inside the heat exchanger tubes, absorbing heat from the outer fluid.
Outer fluid volume: Fluid flowing through the shell, releasing heat to the inner fluid.
With the help of the SimScale platform, we intend to study the heat transfer process occurring in a heat exchanger using the conjugate heat transfer model. The model is based on a strictly mathematically-stated problem, allowing the physical processes and solutions of the governing equations to be considered separately for each object in two subdomains. This makes way for an independent analysis of both the flows (external and internal) and eventually coupling them to plot the effective heat transfer.
A CAD assembly keeping in mind the 3 components as described above has been prepared for the purpose of this project. The images shown below depict the configuration of the fluid volumes and the pipe geometry to be used:
External Geometry of Heat Exchanger
Internal Pipe configuration
When dealing with CHT, we require a multi-region mesh to have a clear definition of the interfaces in the computational domain. Such a mesh can be created with the Hex-dominant parametric operation in the mesh creator.
This Hex-dominant Parametric (only CFD) mesh is used to generate the mesh for the 3 volumes (1 solid and 2 fluids) followed by creating refinements. Also, the volumes are defined as distinct regions in order to define interfaces at a later point in time. Post the meshing operation, we have 3 different regions viz. SolidPipes, InnerFluid, and OuterFluid.
Final Meshed Structure
A laminar steady-state simulation is carried out using the Conjugate Heat Transfer solver. The outer fluid is considered to be at a higher temperature (releasing heat) and the inner fluid at the lower temperature (absorbing heat).
Results and Conclusions
The image below illustrates the flow of the temperature streamlines in the shell of the boiler. As can be seen in the image, the temperature gradient of the Outer fluid is much steeper at entry and gradually decreases with the furtherance of the fluid across the shell.
Streamlines of Temperature (Outer fluid)
The following image displays the temperature contour of the Inner Fluid. The fluid enters from the right-hand side and flows towards the left. The outer fluid transmits heat to the pipe carrying the inner fluid. Subsequently, the inner fluid receives heat through conduction from the pipe through which it flows. As a result, as the flow progresses the area of the red region at the circumferential boundary of the inner fluid increases.
Temperature Contours (Inner fluid)