Heat transfer is the fundamental process by which thermal energy moves from a region of higher temperature to one of lower temperature. This energy exchange is rarely confined to a single material or medium in real-world situations. Conjugate Heat Transfer (CHT) addresses this complexity by modeling the concurrent thermal interaction between a solid structure and the adjacent fluid, whether liquid or gas. This approach treats the combined solid and fluid domains as a single, cohesive thermal system to accurately predict temperature distributions.
Defining Conjugate Heat Transfer
Conjugate Heat Transfer is required for any thermal analysis where the temperature distribution in a solid directly and significantly influences the temperature distribution in the adjacent fluid, and vice versa. It represents the necessity of solving the thermal equations for both domains simultaneously, instead of analyzing the solid and fluid in isolation. The term “conjugate” itself refers to this joining or coupling of two distinct physical domains—the solid and the fluid—at a common interface.
The core difficulty CHT overcomes is that the boundary condition—the temperature or heat flux at the point of contact—must be solved as an unknown part of the overall system. Guessing this boundary condition would result in an inaccurate temperature field in the solid and an incorrect heat transfer calculation in the fluid. Therefore, CHT models couple the thermal physics of both media to determine the temperature and heat flow across the boundary as part of the solution. This integrated method provides a more realistic prediction of the complete thermal behavior of a system than traditional, decoupled methods.
The Mechanism of Interacting Heat Flow
The mechanism of CHT relies on the interaction of the two primary modes of heat transfer that dominate their respective domains. Within the solid structure, heat primarily moves through conduction, which is the transfer of thermal energy via direct molecular vibration and collision. Conversely, within the fluid domain, the movement of heat is dominated by convection, where heat is transported by the bulk motion of the fluid itself. The CHT analysis effectively connects these two different physical processes.
The critical link between the solid and fluid domains is defined by two specific boundary conditions at their shared interface. The first condition is thermal continuity, which dictates that the temperature must be exactly the same on the solid side of the interface as it is on the fluid side at every point. The second condition is the conservation of heat flux, meaning the rate of heat energy leaving the solid must be precisely equal to the rate of heat energy entering the fluid at the interface. This ensures that no heat is artificially gained or lost at the boundary.
The thickness and material properties of the solid component play a significant role in determining the efficiency of the overall heat transfer. A solid with high thermal conductivity, such as a metal, will rapidly spread heat from a source to the solid-fluid interface, making it easier for the fluid to remove the energy. Conversely, a solid with low conductivity acts as a thermal buffer, slowing the heat transfer and allowing the solid to become hotter locally. The fluid’s ability to convect heat away is quantified by its convective heat transfer coefficient. This coefficient is highly dependent on the fluid velocity, density, and turbulence level.
Applications in Engineering and Design
CHT analysis is used across numerous engineering disciplines where precise thermal management is required. One of the most common applications is in the thermal management of modern electronic devices, particularly in cooling high-power chips. CHT helps engineers design effective heat sinks by accurately modeling how heat conducts through the solid fins and then transfers to the cooling fluid, such as air or liquid coolant. Inaccurate modeling in this area can lead to component overheating and premature system failure.
In the aerospace industry, CHT analyzes complex cooling systems for gas turbine blades. These blades are exposed to extremely hot combustion gases, requiring the analysis to couple heat transfer from the hot gas to the blade material with the internal cooling provided by air flowing through channels. Vehicle thermal systems, including engine cooling and exhaust gas recirculation, also rely on CHT to optimize component performance and longevity. This ensures engine components remain within safe operating temperatures while maximizing efficiency.
CHT is also used in the design and optimization of heat exchangers, which transfer heat between two fluids separated by a solid wall. CHT determines the temperature profiles in the solid separating wall and the heat transfer rates for both fluids simultaneously. Failing to account for the conjugate nature of the heat flow results in designs that are inefficient, oversized, or prone to material stress and failure due to localized thermal hotspots.