Large density differences between liquid and vapor create buoyancy effects in the presence of a gravitational field. Such effects can play an important role in two-phase fluid flow and heat transfer, especially critical heat flux (CHF). CHF poses significant risk to electronic devices, and the ability to predict its magnitude is crucial to both the safety and reliability of these devices. Variations in the gravitational field perpendicular to a flow boiling surface can take several forms, from flows at different orientations at 1 g e to the microgravity environment of planetary orbit, to the reduced gravity on the Moon and Mars, and the high g’s encountered in fighter aircraft during fast aerial maneuvers. While high coolant velocities can combat the detrimental effects of reduced gravity, limited power budget in space systems imposes stringent constraints on coolant flow rate. Thus, the task of dissipating the heat must be accomplished with the lowest possible flow velocity while safely avoiding CHF. In this study, flow-boiling CHF is investigated on Earth as well as in reduced gravity parabolic flight experiments using FC-72 as working fluid. CHF showed sensitivity to gravity at low velocities, with microgravity yielding significantly lower CHF values compared to those at 1 g e . Differences in CHF value decreased with increasing flow velocity until a velocity limit was reached above which the effects of gravity became inconsequential. This proves existing data, correlations, and models developed from 1 g e studies can be employed with confidence to design reduced gravity thermal management systems, provided the flow velocity is maintained above this limit. This study discusses two powerful predictive tools. The first, which consist of three dimensionless criteria, centers on determination of the velocity limit. The second is a theoretically based model for flow boiling CHF in reduced gravity below this velocity limit.

This study is the first attempt at extending the Interfacial Lift-off CHF Model to subcooled flow boiling conditions. A new CHF database was generated for FC-72 from ground tests as well as from microgravity tests that were performed in parabolic flight trajectory. These tests also included high-speed video imaging and analysis of the liquid–vapor interface during the CHF transient. Both the CHF data and the video records played a vital role in constructing and validating the extended CHF model. The fundamental difference between the original Interfacial Lift-off Model, which was developed for saturated flow boiling, and the newly extended model is the partitioning of wall energy between sensible and latent heat for subcooled flow boiling. This partitioning is modeled with the aid of a new “heat utility ratio”. Using this ratio, the extended Interfacial Lift-off Model is shown to effectively predict both saturated and subcooled flow boiling CHF in Earth gravity and in microgravity.

A comprehensive review and analysis of prior subcooled flow boiling CHF correlations was conducted to identify those correlations that provide the most accurate predictions for dielectric working fluids and small rectangular flow passages found in electronics cooling applications in both microgravity and Earth gravity. Since most prior correlations were derived from water databases, only those with dimensionless form were deemed potentially suitable for other working fluids. Only a small fraction of these dimensionless correlations were found to tackle other fluids and more complicated flow and heating configurations with acceptable accuracy. These correlations were ranked relative to mean error, mean absolute error, and root mean square error. Better predictions where achieved when correlations were based on the heated diameter rather than the hydraulic diameter because of the ability of the former to better describe vapor development in subcooled flow. Two previous correlations by Hall and Mudawar provided the best overall CHF predictions for both microgravity and Earth gravity.

This study examines both high-flux flow boiling and critical heat flux (CHF) under highly subcooled conditions using FC-72 as working fluid. Experiments were performed in a horizontal flow channel that was heated along its bottom wall. High-speed video imaging and photomicrographic techniques were used to capture interfacial features and reveal the sequence of events leading to CHF. At about 80% of CHF, bubbles coalesced into oblong vapor patches while sliding along the heated wall. These patches grew in size with increasing heat flux, eventually evolving into a fairly continuous vapor layer that permitted liquid contact with the wall only in the wave troughs between vapor patches. CHF was triggered when this liquid contact was finally halted. These findings prove that the CHF mechanism for subcooled flow boiling is consistent with the interfacial lift-off mechanism proposed previously for saturated flow boiling.

Poor understanding of flow boiling in microgravity has recently emerged as a key obstacle to the development of many types of power generation and life support systems intended for space exploration. This study examines flow boiling CHF in microgravity that was achieved in parabolic flight experiments with FC-72 onboard NASA’s KC-135 turbojet. At high heat fluxes, bubbles quickly coalesced into fairly large vapor patches along the heated wall. As CHF was approached, these patches grew in length and formed a wavy vapor layer that propagated along the wall, permitting liquid access only in the wave troughs. CHF was triggered by separation of the liquid–vapor interface from the wall due to intense vapor effusion in the troughs. This behavior is consistent with, and accurately predicted by the Interfacial Lift-off CHF Model. It is shown that at low velocities CHF in microgravity is significantly smaller than in horizontal flow on earth. CHF differences between the two environments decreased with increasing velocity, culminating in virtual convergence at about 1.5m/s. This proves it is possible to design inertia-dominated systems by maintaining flow velocities above the convergence limit. Such systems allow data, correlations, and/or models developed on earth to be safely implemented in space systems.

Experiments were performed to examine the effects of body force on flow boiling CHF. FC-72 was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface just prior to CHF. High-speed video imaging techniques were used to identify dominant CHF mechanisms corresponding to different flow orientations and liquid velocities. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed significant sensitivity to orientation for flow velocities below 0.2 m/s, where extremely low CHF values where measured, especially with downward-facing heated wall and downflow orientations. High flow velocities dampened the effects of orientation considerably. The CHF data were used to assess the suitability of previous CHF models and correlations. It is shown the Interfacial Lift-off Model is very effective at predicting CHF for high velocities at all orientations. The flooding limit, on the other hand, is useful at estimating CHF at low velocities and for downflow orientations. A new method consisting of three dimensionless criteria is developed to determine the minimum flow velocity required to overcome body force effects on near-saturated flow boiling CHF.

Vertical upflow boiling experiments were performed in pursuit of identifying the trigger mechanism for subcooled flow boiling critical heat flux (CHF). While virtually all prior studies on flow boiling CHF concern the prediction or measurement of conditions that lead to CHF, this study is focused on events that take place during the CHF transient. High-speed video imaging and photomicrographic techniques were used to record the transient behavior of interfacial features from the last steady-state power level before CHF until the moment of power cut-off following CHF. The video records show the development of a wavy vapor layer which propagates along the heated wall, permitting cooling prior to CHF only in wetting fronts corresponding to the wave troughs. Image analysis software was developed to estimate void fraction from the individual video images. The void fraction records for subcooled flow boiling show the CHF transient is accompanied by gradual lift-off of wetting fronts culminating in some maximum vapor layer mean thickness, following which the vapor layer begins to thin down as the transition to film boiling ensues. This study proves the interfacial lift-off model, which has been validated for near-saturated flow boiling CHF, is equally valid for subcooled conditions.

Experiments were performed to examine the effects of body force on flow boiling CHF. FC-72 was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface just prior to CHF. High-speed video imaging techniques were used to identify dominant CHF mechanisms. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed significant sensitivity to orientation for flow velocities under 0.2 m/s, where extremely low CHF values where measured, especially with a downward-facing heated wall and downflow orientations. High flow velocities dampened the effects of orientation considerably. The CHF data were used to assess the suitability of previous CHF models and correlations for different orientations and velocities. It is shown the Interfacial Lift-off Model is very effective at predicting CHF for high velocities at all orientations. The flooding limit, on the other hand, is useful at estimating CHF at low velocities and downflow orientations.

The effects of orientation on flow boiling critical heat flux (CHF) were investigated using high-speed video and microphotographic techniques. Interfacial features were measured just prior to CHF and statistically analyzed. A dominant wavy vapor layer regime was observed for all relatively high-velocities and most orientations, while several other regimes were encountered at low velocities, in downflow and/or downward-facing heated wall orientations. The interfacial lift-off model was modified and used to predict the orientation effects on CHF for the dominant wavy vapor layer regime. The photographic study revealed a fairly continuous wavy vapor layer travelling along the heated wall while permitting liquid contact only in wetting fronts, located in the troughs of the interfacial waves. The waves, which were generated at an upstream location, had a tendency to preserve a curvature ratio as they propagated along the heated wall. CHF commenced when wetting fronts near the outlet were lifted off the wall. This occurred when the momentum of vapor normal to the wall exceeded the pressure force associated with interfacial curvature. The interfacial lift-off model is shown to be very effective at capturing the overall dependence of CHF on orientation.

The interfacial instabilities important to the modeling of critical heat flux (CHF) in reduced-gravity systems are sensitive to even minute body forces, especially for small coolant velocities. Understanding these effects is of paramount importance to both the reliability and safety of two-phase thermal management loops proposed for future space and planetary-based thermal systems. Unfortunately, reduced gravity systems cannot be accurately simulated in 1g ground-based experiments. However, ground-based experiments can help isolate the effects of the various forces (body force, surface tension force and inertia) which influence flow boiling CHF. In this project, the effects of the component of body force perpendicular to a heated wall were examined by conducting 1g flow boiling experiments at different orientations. Boiling experiments were performed using FC-72 in vertical and inclined upflow and downflow, as well as horizontal flow, and with the heated surface facing upward or downward relative to gravity. CHF was very sensitive to orientation for flow velocities below 0.2 m/s and near-saturated flow; CHF values for downflow and downward-facing heated surface were much smaller than for upflow and upward-facing surface orientations. Increasing velocity and subcooling dampened the effects of flow orientation on CHF. For saturated flow, the vapor layer characteristics fell into six different regimes: wavy vapor layer, pool-boiling, stratification, vapor stagnation, vapor counterflow, and vapor concurrent flow. The wavy vapor layer regime encompassed all subcooled and high-velocity saturated conditions at all orientations, as well as low-velocity upflow orientations. Prior CHF correlations and models were compared, and shown to predict the CHF data with varying degrees of success.

The bubble coalescence and interfacial instabilities that are important to modeling critical heat flux (CHF) in reduced-gravity systems can be sensitive to even minute body forces. Understanding these complex phenomena is vital to the design and safe implementation of two-phase thermal management loops proposed for space and planetary-based thermal systems. While reduced gravity conditions cannot be accurately simulated in 1g ground-based experiments, such experiments can help isolate the effects of the various forces (body force, surface tension force and inertia) which influence flow boiling CHF. In this project, the effects of the component of body force perpendicular to a heated wall were examined by conducting 1g flow boiling experiments at different orientations. FC-72 liquid was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface at conditions approaching CHF. High-speed video imaging was employed to capture dominant CHF mechanisms. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed great sensitivity to orientation for flow velocities below 0.2 m/s, where very small CHF values where measured, especially with downflow and downward-facing heated wall orientations. High flow velocities dampened the effects of orientation considerably. Figure I shows representative images for the different CHF regimes. The Wavy Vapor Layer regime was dominant for all high velocities and most orientations, while all other regimes were encountered at low velocities, in the downflow and/or downward-facing heated wall orientations. The Interfacial Lift-off model was modified to predict the effects of orientation on CHF for the dominant Wavy Vapor Layer regime. The photographic study captured a fairly continuous wavy vapor layer travelling along the heated wall while permitting liquid contact only in wetting fronts, located in the troughs of the interfacial waves. CHF commenced when wetting fronts near the outlet were lifted off the wall. The Interfacial Lift-off model is shown to be an effective tool for predicting the effects of body force on CHF at high velocities.