Impact of heat transfer due to fiber conduction, radiation and convection on the interpretation of experiments with isolated droplets

Isolated droplet setups have been extensively used during the last decades to characterize liquid fuel evaporation and combustion, being considered a reference framework approaching the simplest and idealized (canonical) configuration. However, in many cases, this configuration is affected by different experimental artifacts that may lead to non-negligible deviations from the assumed ideal conditions and must be duly assessed in order to correctly interpret experimental results. The conduction of heat through the support fibers, the absorption of thermal radiation and external convective effects are specifically studied in this work. A theoretical analysis addresses these undesired effects, proposing a dimensionless number for each artifact (FN, RN, CN, respectively), accounting each of them for the ratio of the corresponding additional heat input when compared to the canonical case. For moderate values of these heat inputs, a linear correlation is proposed, where the deviation of the droplet evaporation rate from the canonical case (K/Kcan) can be predicted solely based on FN, RN and CN. This correlation has been validated for a wide range of conditions using a droplet evaporation model as well as experiments performed on two different test rigs (drop-tube and suspended droplet setups). Such a broad variety of conditions (in terms of initial droplet diameter, size and material of the suspension fibers, fuel, gas temperature, etc.) is considered to provide a unique and novel dataset with a rich assortment of data, ranging from tests very close to the canonical case to experiments where the artifacts completely distorted the evaporation behavior, surging the evaporation rate and even inducing internal bubbling and puffing events. Additional data from the literature was also examined, further supporting the ability of the proposed approach to accurately capture the deviations in terms of evaporation rate due to these artifacts.