A Novel Heat Exchanger Effectiveness Approach for Exploring Performance Limits of Two-Phase Mini-Channel Boilers

Indirect two-phase cooling with refrigerants allows management of CPU/GPU heat loads well in excess of those that can be managed with water cooling. Two-phase mini channel boilers are at the heart of such systems. Similar to single-phase cold plates, the performance of mini-channel boilers depends on proper selection of design parameters such as fin pitch, thickness and total surface area that ultimately dictates the number of fins. But unlike single-phase cold plates, two-phase cold plates must also take into account ancillary issues such as boiling flow instabilities, vapor generation and potential loss of coolant that severely affect thermal performance. The traditional metric used in assessing the thermal performance of boilers is thermal resistance, which can be used for comparative studies but does not offer a true assessment of cooling limits. In this paper, we introduce a new definition of heat exchanger effectiveness for two-phase boilers that can be used to compare actual performance to maximum performance and by its definition can also establish cooling limits. We show that the thermal resistance is directly related to the effectiveness. By first maximizing effectiveness, the design thermal resistance can be achieved by the proper sizing of an optimized heat exchanger core. The methodology is first validated for single-phase liquid cold plates by comparing the thermal resistance obtained from the proposed approach to that obtained from the commonly used definition. The proposed approach is demonstrated on experimentally characterized boilers found in the literature. The experimentally determined effectiveness for these test boilers demonstrates the utility of the technique.