Fig. 4: Impact of natural convection and insulating layer thickness on thermal conductivity measurements.
From: Accurate and rapid measurement of fluid thermal conductivity
a Numerical simulation of the Rayleigh number (Ra) as a function of channel aspect ratio and vertical temperature difference for five fluids: water, ethanol, octane, toluene, and diethyl ketone. The critical Ra range is identified between 200 and 400, indicating the onset of natural convection in the 3D channel. b Influence of natural convection on thermal conductivity measurements using water as the reference fluid. Convection starts to play a role in the largest channel (750 µm) at ~400. c Insensitivity of the thermal conductivity measurement approach to natural convection, if present. d Simulated heater temperatures as a function of the liquid thickness for various polyamide insulating layers, ranging from 40 µm to 100 µm, using water as the reference fluid and propylene glycol as the sample fluid. All temperatures increase uniformly as the thickness of the insulating layer is increased, indicating that the derived thermal conductivity remains almost constant. e The plot shows the thermal conductivity derived from the simulations as the thickness of the polyamide layer increases. The error is <1% in all cases. Source data are provided as a Source Data file.
