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Transport-related phenomena for clusters of drops

Bellan, J. and Harstad, K. (1989) Transport-related phenomena for clusters of drops. International Journal of Heat and Mass Transfer, 32 (10). pp. 2000-2002. ISSN 0017-9310. doi:10.1016/0017-9310(89)90253-6.

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Measurements performed in sprays characteristic of power systems show that sprays are composed of several regions [1]. Near the atomizer the drops might not be entirely formed and liquid sheets and filaments might still exist. There follows a region where the drops are already formed but have not yet been dispersed, so that they cluster together with a typical distance between the drops that is of the same order of magnitude as that of the average radius of the drops themselves. This region of the spray is called the dense spray region. Finally, further from this dense spray region there exists a region where the drops might still cluster, but in these clusters the distance between drops is much larger than the average radius of the drops. This region is called the dilute spray region. In the dilute spray region drops are far apart from each other and thus when the spray is exposed to a convective flow, these drops practically behave like isolated drops in a convective flow. In contrast, in the dense spray regime, the drops are close to each other and thus their history is controlled by how much of the surrounding gas can enter in contact with them. This is to say that, unlike for drops belonging to dilute clusters of drops, transport phenomena are crucial in determining the behavior of drops belonging to dense clusters of drops because transport imposes limits on heat and mass transfer between the two phases. These phenomena pertain to indirect interactions and they can control the motion of drops, their heat-up time, evaporation, ignition and combustion. Previous work [2-5] pointed out some important consequences of these indirect interactions. Two models of turbulent transport were used in ref. [5] in order to investigate the importance of turbulent transport from the surroundings to the cluster. Because of the global aspect of the model in which all the drops were assumed to behave identically, the transport from the cluster to surroundings was modeled using a 'trapping factor'. Basically, the 'trapping factor' is a weighing factor which allows the modeling of intermediary situations between those of dilute clusters where evaporated mass was assumed to be trapped in the cluster and that of dense clusters where evaporated mass was assumed to escape to ambient. It was found [5] that whereas in the dilute regime turbulence is not a controlling parameter, in the dense regime it becomes the crucial control parameter. This is a fact well known by experimentalists and design engineers who locate turbulent enhancement devices near the injector where the spray is dense, rather than further down the combustor where the spray is dilute. Since the transport processes between the cluster and its surroundings were found to be so important in the case of dense clusters, it was thought very important to improve the description of the transport of heat, mass and species from the cluster and its surroundings. This new model is described in detail in ref. [6] for electrostatically charged drops, and is used to calculate the results presented below for the special case of null charge. Due to the brief nature of the Technical Note, the nomenclature used here is the same as in refs. [5, 6]. The model developed in ref. [6] is similar to that of ref. [5] in that the drops and gas have two velocity components: a uniform axial component along the trajectory direction and a radial component. The difference between the two models is in the description of the radial velocity component. Whereas in ref. [5] a 'trapping factor' was used as discussed above, the new formulation uses the assumption of self-similarity in the radial direction as explained in detail in ref. [6].

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Bellan, J.0000-0001-9218-7017
Additional Information:© 1989 Elsevier Ltd. Received 31 October 1988, Accepted 3 April 1989.
Issue or Number:10
Record Number:CaltechAUTHORS:20171025-104133863
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Official Citation:J. Bellan, K. Harstad, Transport-related phenomena for clusters of drops, In International Journal of Heat and Mass Transfer, Volume 32, Issue 10, 1989, Pages 2000-2002, ISSN 0017-9310, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:82654
Deposited By: Tony Diaz
Deposited On:25 Oct 2017 18:44
Last Modified:15 Nov 2021 19:52

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