Unsteady conjugate heat transfer effects on flow characteristics in transonic flow

We present a detailed study into compressible flows in presence of unsteady conjugate heat transfer to the walls. The investigation reveals that all flow quantities experience drift until thermal steady state is achieved in both fluid and solid domains. The drift follows a characteristic exponential decay, and we demonstrate that the heat transfer process between the two domains is ruled by the ratio of thermal capacity and the Stanton number that characterises the interface. To illustrate these effects, we numerically analyse the influence of slow thermal transients due to conjugate heat transfer on discharge characteristics of non-adiabatic nozzles. A transonic nozzle with thermally conducting walls is simulated using numerical methods with different fidelity, from one-dimensional reduced-order models to large eddy simulations, and the transient evolution of mass flow rate is monitored. Additionally, it is observed that while preheating does not affect the decaying time constant of the drift, the bounds of the decay are directly proportional to the initial temperature difference. • In unsteady CHT, flow quantities are drifting until thermal steady state is reached. • Thermal transient can be significant in cases of high initial temperature gradients. • Preheating the solid reduces the bounds of the thermal drift. • Reduced order models can provide order of magnitude estimates of the transient.