Ultra-high critical heat flux (CHF) data, with many values exceeding 100 MW m−2, were obtained using high mass velocity, subcooled water flow through short, small diameter tubes. These tests produced the highest CHF of 276 MW m−2 reported in the literature for a uniformly heated tube which surpassed the prior record of 228 MW m−2. The data include broad ranges of tube diameter (0.406–2.54 mm) , heated length-to-diameter ratio (2.4–34.1) , mass velocity (5000–134 000 kg m−2 s−1) , inlet temperature (18–70°C) , and outlet pressure (2.5–172.4 bars) . The parametric trends of CHF were ascertained relative to all important flow and geometrical parameters. CHF increased with increasing mass velocity, increasing subcooling, decreasing tube diameter, and decreasing heated length-to-diameter ratio. For a constant inlet temperature, CHF increased with increasing pressure for pressures up to 30 bars, remained fairly constant between 30 and 150 bars, and decreased afterwards as the critical pressure was approached. CHF was accompanied by physical burnout of the tube wall near the exit and tube material had little effect on the magnitude of CHF. The pressure drop for most conditions was fairly constant, albeit as high as 153.4 bars, over the entire range of heat fluxes, from the single-phase flow condition corresponding to zero heat flux up to CHF, proving that CHF was triggered even with negligible net vapor production. These high pressure drops indicate special attention should be exercised when employing the high mass velocity flows necessary to attaining ultra-high CHF. These high pressure drops also render the practice of referencing CHF data reSlative to a single measured pressure value very misleading.

A data base was obtained for ultra-high CHF, with values mostly exceeding 104 W cm-2, using high mass velocity water flow through small diameter tubes. The parametric trends of ultra-high CHF were ascertained with respect to key flow and geometrical parameters over a wide range of exit pressures. Pressure drop for most conditions was fairly constant over the entire range of heat fluxes, from the single-phase flow condition corresponding to zero heat flux up to CHF, proving CHF was triggered even with negligible net vapor production. CHF increased with increasing mass velocity, increasing subcooling, decreasing tube diameter, and decreasing heated length. CHF increased with increasing pressure up to about 30 bars, became fairly constant up to about 100 bars, and then decreased for higher pressures. A new CHF correlation was developed for pressures ranging from 10.3 to 172.4 bars corresponding to CHF values as high as 2.76 × 104 W cm-2.

Due to the need for practical cooling technologies which could dissipate high heat fluxes, an experimental study of pressure drop and CHF in mini-channel (D = 2.54 mm) and micro-channel (D = 510 μm) heat sinks of 1 cm heated length was performed using R-113. Test conditions included inlet subcooling ranging from 10 to 32°C and a range of low flow rates up to a maximum of 95 ml min−1. The tests yielded CHF values for both heat sinks in excess of 200 W cm−2 with the advantage of both low flow rates and low pressure drops (ΔP < 0.32 bar) as compared to high-flux, single-phase micro-channel heat sinks. Key features of the miniature heat sinks include a lack of inlet subcooling effect on CHF and superheated outlet conditions at the lowest flow rates. A single CHF correlation was developed for both heat sinks. The paper will illustrate the use of the CHF correlation and a generalized model for pressure drop as predictive tools in assessing the merits of different channel sizes in incorporating miniature heat sink technology into high heat flux cooling schemes. Overall, the mini-channel's performance proved superior to the micro-channel due to pressure drops less than 0.01 bar for comparable CHF values as well as the reduced likelihood of clogging and the relative ease in fabricating the mini-channel.

Increased rate of heat dissipation from electronic chips was explored by the application of flow boiling in mini-channel (D = 2.54 mm) and micro-channel (D = 510 μm) heat sinks with special emphasis on reducing pressure drop and coolant flow rate. A pressure drop model was developed that accounts for the single-phase inlet region, the single- and two-phase heated region, and the two-phase unheated outlet region. Inlet and outlet losses associated with the abrupt contraction and expansion, respectively, were also accounted for, and so were the effects of compressibility and flashing within the two-phase region. Overall, the major contributor to pressure drop was the acceleration caused by evaporation in the channels; however, compressibility effects proved significant for the micro-channel geometry. Based upon practical considerations such as pressure drop, erosion, choking, clogging, and manufacturing ease, the mini-channel geometry was determined to offer inherent advantages over the micro-channel geometry. The latter is preferred only in situations calling for dissipation of high heat fluxes where minimizing weight and liquid inventory is a must.