Laboratory scale analysis of the influence of different heat transfer mechanisms on liquid nitrogen vaporization rate

The prediction of the potential hazards associated to accidental liquefied natural gas (LNG) spills has motivated a number of different studies including experimental and numerical approaches. Most of these studies focus on dispersion predictions, however there is limited information regarding source term of it: liquid spill and vaporization. There is a need of further improvements on the understanding of these phenomena and the quantification of the most important parameters that can affect them. The vaporization of cryogenic liquids is governed by the heat transfer phenomena including conduction, convection and thermal radiation mechanisms. The present work investigates the contribution of each of these heat transfer modes to the vaporization rate of cryogenic liquid nitrogen (LN₂) contained in a Dewar flask using well controlled and instrumented laboratory scale experiments. LN₂ vaporization rate was measured with individually controllable contributions from convective (generated by an electric fan) and thermal radiative (generated by light bulb) heat transfer in the presence of a baseline conductive heat transfer rate. In both cases of convection and radiation analysis the experimental study showed that they can play a significant role in the vaporization rate of LN₂. It was observed that the radiative heat absorbed by the LN₂ during the vaporization experiment represents only 50%-65% of the incident radiation that would reach the LN₂ pool surface if no vapour was present. Convective heat transfer generated by the fan was shown to have had the most significant contribution to the total heat transfer. As expected, this contribution was significantly higher than the one from bulb radiation. The experimental data also showed that the liquid level in the Dewar play a key role in the resulting amount of convective heat transfer. This could be attributed to the fact that lower liquid level the side walls of the Dewar were high enough to hold a layer of vapour and limit air motion directly above the liquid surface, thus limiting the heat transfer by convection.