An experimental investigation of a scaled-down and simplified laboratory model of a transformer cooling system is presented and discussed in this paper. The overall goal of this work was to obtain experimental data for validating cost-effective approximate mathematical models, used in investigations aimed at the development of numerical methods for assessing the operating limits of current transformers and optimizing the designs of next-generation transformers. The laboratory model used in this work was a vertical, single-phase, closed-loop thermosyphon operating with water as the working fluid. The steady and unsteady behaviors of this closed-loop thermosyphon were established and investigated using the following series of power inputs: 50 W, 125 W, 200 W, 125 W, and 50 W. Each of these levels of power input was maintained until steady-state conditions were achieved; and then the excursion to the adjacent power level (up or down, depending on the position in the aforementioned series) was effected. The corresponding experimental results are presented and discussed in this paper. The steady-state experiments with water as the working fluid are used to obtain valuable benchmarking results and also reliable initial conditions for the unsteady experiments. The experiments with excursions from one power level to an adjacent one provide novel results pertaining to unsteady operation of closed-loop thermosyphons. Another novel feature of this work is a demonstration that a simple lumped-parameter formulation can yield good predictions of the overall unsteady behavior of closed-loop thermosyphon systems akin to those used for cooling transformers.

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