Boulder boiling heat transfer

MICROSCALE PHASE CHANGE HEAT TRANSFER TO WATER

Robert H. Leyse*

INZ Inc., P. O. Box 2850, Sun Valley, ID 83353

bobleyse@aol.com

Abstract

   Leyse has pioneered the field of microscale phase change heat transfer to water at ultra-high power density with fine platinum wires, 7.5 μm diameter, that are joule heated in pressurized degassed deionized water.  Each wire functions simultaneously as a heat transfer element and as a resistance thermometer as originated by Nukiyama (1934). These experiments cover the pressure range from 200 to 4000 PSIA and the heat flux range from very low to 4000 W/cm² while bulk water temperature is maintained in the range of 20 ^oC.  These investigations cover two separate situations: Case (i) constant pressure and varying power, and Case (ii) somewhat constant power and varying pressure.  Limited explorations reveal a significant impact of dissolved nitrogen (saturated) at 1000 PSIA.

introduction

   One reviewer of an early Leyse paper remarked, “The study has used interesting ultra high heat fluxes over a wide pressure range. New transition paths are demonstrated in the results. The author does not claim fundamental explanations of the phenomena, but he challenges the theoreticians as well as the numerical modelers to use their skills instead for further study of the phenomena. However, because the report contains new results that are interesting enough to excite new theories in the field of boiling phenomena, I recommend its publication.”  To date neither Leyse nor anyone else has come forth with related new theories in the field of boiling phenomena.

APPARATUS AND PROCEDURES

   The heat transfer element is a fine platinum wire, 7.5 μm diameter by 1.14 mm long. It is installed within the lower end of a vertical stainless steeel tube,  0.3 inches internal diameter. by 8 inches long.  The arrangement of the  platinum wire and ancillary support apparatus is detailed in Figure 1 in which the pyrex tube is repalced by the stainless steel tube for the high pressure runs of this presentation.  The microscale heat transfer element is welded to platinum terminals, 0.020 inch diameter.  The local water temperature is measured with a chromel-alumel thermocouple that is sheathed withn a 0.020 inch diameter stainless steel assembly.   This thermcouple junction  is about 0.020 inches below the horizontal fine platinum wire. The assembly is filled with degassed, demineralized water and is pressurized with a pneumatic hand pump.  Pressure is monitored with a Rosemount recording pressure transducer .

   The heat transfer element is powered with a programmed direct current power source.  The power measurement is atraightforward.  A 10 ohm precision resistance is in series with platinum element.  Voltage is measured across the platinum element and across the 10 ohm resistance.  The product of these two voltages divided by 10 yields the power in watts to the platinum element.  All data, the two voltages, the pressure, and the water temperature  is recorded every 0.1 second  in an excel spreadsheet which facilitates data handling, plotting and analysis. 

   The programmed power supply controls the total voltage drop across the two resistances.  This is a satisfactory arrangement for the runs in which power is programmed during the series of runs at constant pressure, Case (i).  As will be discussed later, it is an acceptable, although possibly a less satisfactory arrangement for runs at somewhat constant heat flux, Case (ii). (The runs at somewhat constant heat flux were an afterthought; a worthwhile afterthought).  A run at constant pressure is completed in about 10 seconds, the power increases for 5 seconds and then decreases for 5 seconds (Figure 2).

   Preparations for a run consist of equipment setup incluidng the fillng and venting processes to insure a “solid” system with degassed, demineralized water.  The programmed power supply is set (checked) for a linear increase and decrease to a  with the 10 ohm restsistor and a dummy resistor, usually three ohms.  Following these preparations,  the run proceeds; data are  recorded every 0.1 seconds as power increases for about 5 seconds and then decreases for aboout 5 seconds and the run is complete.                                                               

 

runs at constant pressure and varying power

   The top plot in Figure 3 covers all of  the data for eight runs at constant pressure ranging from 200 to 6000 PSIA.      At subcritical pressures 200, 1000 and 2500 PSIA, the inception of nucleate boiling begins at progressively increasing temperatures,  240, 307 and 349 ^oC.  The plots are nearly identical for the increasing and decreasing data sets.  (At 3000 PSIA there is a unique situation that is detailed in a  later discussion, Figures 4 and 5.)

   At supercritical pressures there is completely different characteristic.  The temperature of the heat transfer element increases vastly with only a modest inclrease in heat flux, in contrast to the nucleate boiling charactersitc at subcritcial presssures where only a modest increase in temperature yields a substantial increase in heat flux.. At 6000 PSIA the increasing and decreasing plots are almost identical beyond 550 ^oC, and are somewhat together over their entire span.  The divergence between increasing and decreasing data sets progressively increases for runs at the pressure data sets from 6000, 5000, 4300  to 3600 PSIA.

   The plots for the increasing power only are the mid set in Figure 3.  Note that the departures from natural circulation are somewhat similar for the runs at 3600 and 4300 PSIA in the temperature range from  the critical temperature to about 550 ^oC.  This is in contrast to the runs at 5000 and 6000 PSIA. The 3000 PSIA case is clearly distinct.   

   The consistently parallel plots in the lower set in Figure 3also  include the 3000 PSIA case.  It is interesting that at all pressures, the return to natural circulation without phase change heat transfer is at progressively lesser subcritical temperatures, although the 6000 PSIA case  is very close to the critical  temperature.  It is also interesitng that the 3000 PSIA case fits very well into the trends.

   See Figure 4 for futher discussion of the subcritical data.  Note that the transition to phase change  heat transfer is 65 ^oC greater than the saturation temperature at 200 PSIA and that it becomes very little at 2500 PSIA. Moving to Figure 5, at 3000 PSIA there is a complex transiation to phase change heat transfer.   At a heat flux of about 2700 W/cm²  there is a transition to nucleate  boiling at very nearly the saturation temperature.  Within about 0.3 seconds there is a departure from the nucleate boiling.  And, within another  0.1 seconds the peak temperature jumps by about 480 ^oC to  876^oC.  The 876 ^oC point is at the peak of the programmed power.  The return to non-phase change heat transfer proceeds more slowly, about 0.9 seconds.  The return is at 300 ^oC and that is about 60 ^oC less than the departure.

runs at constant pressure and varying power with dissolved nitrogen

   The apparatus also has been operated as described with the water saturated with disolved nitrogen.  A supply of nitrogen under high pressure was valved to the apparatus for several days at 1000 PSIA such that the water became saturated with disolved nitrogen.  Leyse discovered that the presence of dissolved nitrogen changed the heat transfer during non phase change heat transfer (natural circulation).  It also alterted the point of departure to phase change heat transfer. He received a U. S. Patent for his process. The plots in Figure 6 are copied from the patent. The following text is copied from the patent; the refernces to FIG. 4 and FIG. 5 are for the plots in Figure 6.

   The plot of power applied versus resistance of the sensor element when immersed in degassed water at approximately 1000 psi is shown in FIG. 4.  A corresponding plot of water saturated with nitrogen at approximately 1000 psi shown in FIG. 5 reveals several aspects which quantify the presence of dissolved nitrogen in the nitrogen saturated water relative to the degassed water.  Each curve has a region of linear increasing slope S and a knee K at which the slope abruptly increases.  With degasssed water the linear slope S is 0.26 watts/ohm while for water saturated with nitrogen the slope S is 0.19 watts/ohm.  With degassed water, the coordiantes of the knee K are 8.7 ohms and 0.97 watts while with water saturated with nitrogen, the coorddinates of the knee are 8.0 ohms and 0.66 watts.  Similar calibrations may be produced for intermediate concentrations of dissovled nitrogen.

        It is not necessary to present this discovery as plots of heat flux vs. temperature in order to have an operational and patentable device.  However, it is clear that the heat transfer coefficient during natural circulation is substantially less with dissolved nitrogen.  It is also clear that the transition from natural circulation to phase change heat transfer occurs at a substantially lower heat flux for the case with dissolved nitrogen.  It appears that the transition from natural circulation to phase change heat transfer has a somewhat rounded shape with dissolved nitrogen in contrast to the relatively sharp transition with degassed water. This apparent difference in the shape of the transition very likely has no practical applications; however, it should be of interest to “theoreticians and numerical modelers.”  I propose to substitute, “The apparent difference in the shape of the transition is under investigation (Appendix A, Application of MOOSE)

Figure 3  Microscale Phase Change Heat Transfer to Subcooled Water – Three Plots

Runs at substantially constant power and varying pressure

   These runs were completed as an afterthought.  Each of four runs has the total voltage set at a fixed value.  (Recall that the total voltage is the sum of that across the 10 ohm resistance plus that across the platinum element.)   A run proceeds as follows:  The apparatus is pressurized to about 6000PSIA, power is turned on, and pressure is steadily reduced to about 200 PSIA over a period of about 20 seconds. See Figures 7 and 8.  Figure 7 is a plot of heat flux during each run; the runs are coded according to the peak heat flux in each; 2630, 2930, 3010 and 3360 W/cm².  Figure 8 is a plot of  the corresponding temperature of the heat transfer element during each run.

   For run 2630, the plot is smooth over the entire span and the heat flux is within 30 W/cm² of 2600 W/cm2 for the span from about 6000 PSIA to 1000 PSIA.  The corresponding temperature plot, Figure 8, is also smooth and relatively flat.

   Runs 2930 and 3010 each have a distinct upward step of about 200 ^oC at about 3800 and 4200 PSIA respectively. Next, each has a steep temperature increase up to the critical temperature at which point the temperature “turns around” and a steep decrease follows until there is a step decrease at about 2400 PSIA for each.  The step decreases each terminate very near to the saturation temperature.

   Run 3360 has the same character as runs 2930 and 3010, although the upward step is at a higher pressure than is covered in these investigations.

   The heat flux plots, Figure 7, reflect the varying resistance of the platinum element.  An increase in resistance of the element (temperature) leads to an increased voltage drop and an increased power.  This is a consequence of the control by fixed voltage across the series circuit of the 10 ohm resistance and the platinum element.  As the resistance of the platinum element increases, its fraction of the total voltage increases.  Although this leads to less amperage in the circuit, the net impact is an increase in power to the platinum element.  The circuit design turns out to be fortuitously tailored for this investigation because if the heat flux was indeed held constant over the pressure range it would take a multitude of runs to discover the step changes as well as the turnaround at or near the critical pressure.

   In Figure 8 the plots of all runs are very close together at pressures from about 1400 PSIG to termination of the runs at about 300 PSIG.  This because the heat transfer is by nucleate boiling, and at any given pressure the temperature varies relatively little with heat flux.  Clearly, the plots would be very similar to these even if constant heat flux had been achieved over that pressure range.   This is consistent with the plots in Figure 4 in two respects; one, at any given pressure the temperature varies relatively little with heat flux, and two, the difference between the platinum temperature and the saturation temperature decreases as the saturation temperature increases.

Apparatus FOR MONITORING the circulation patterns

   Apparatus is visualized for determining the circulation patterns.  Two phases are planned.  The first phase will utilize the present assembly with two opposed elements.  One element will be will powered and the opposite element monitor temperature.  The test procedure will include pulsed application of high heat flux and precise timing of the response of the temperature element.  The data from this apparatus, especially the precision timing data, will be applied to check out MOOSE. (APPENDIX A).

   The second phase will include a complex assembly with perhaps eight platinum elements.  One element will be the heat transfer element while others will be resistance thermometers.  At least two of the elements will be vertical.  A series of runs will have the heat transfer source rotated among the elements.  Circulation patterns will be inferred via data analysis.  One challenge is to support and deliver power to the fine platinum wires without unduly disturbing the flow patterns; fine copper wires under tension may be feasible.

   Details of the campaign are incomplete.  These phase two activities will be iterative; a series of assemblies will be modified from run to run, etc.  The MOOSE analysis will significantly impact the arrangements of the platinum elements.   The  analysis and application of the measurements will also lead to adjustments of MOOSE.

   Measurements to date have been at 0.1 second intervals.  Future capability will include 0.001 second intervals and perhaps faster.  A faster recording capability will yield further detail of at least three aspects of the runs to date; the jump from about 480 ^oC to  876^oC at 3000 PSIA, the increasing and the decreasing step changes during the runs at substantially constant power, and the sharpness of the transitions from natural circulation to nucleate boiling or supercritical conditions.

   The run series will include at least six features:

1.     Ramped runs with durations including and exceeding the ten seconds of runs to date.

2.     Runs with step power inputs up to 4000 W/cm² and precise timing of responses of temperature detectors.

3.     Runs at system pressures from one atmosphere to 400 atmospheres.

4.     Runs at system bulk temperatures form 20^oC to 370^oC.

5.     Runs with step power input up to 4000 W/cm² and controlled uniform rate of system pressure decrease from 6000 PSIA to 200 PSIA over 10 seconds and 20 seconds.

6.     Runs designed to improve this application of MOOSE.

 

COMMENTS

   At subcritical pressures this work did not explore heat transfer regimes beyond nucleate boiling. At the time it was believed that achieving 4000 W/cm² at 1000 PSIG was remarkable.  Leyse believed that burnout was less likely at one third of the critical pressure and therefore the programmed power was restricted to lesser peak heat fluxes at higher subcritical pressures. The results at 3000 PSIA with the brief time, about 0.3 seconds, in the nucleate boiling regime, followed by the fantastic jump across the supercritical temperature arena with very little increase in power justifies the tame approach in setting peak voltage.  The exploration of the supercritical arena also proceeded with caution; however, 4000 W/cm² was achieved at 6000 PSIA.

     Finally, Bakhru and Lienhard, 1972, disclosed in their publication, BOILING FROM SMALL CYLINDERS that, “Nucleate boiling does not occur on the small wires” and “Three modes of heat removal are identified

for the monotonic curve and described analytically: a natural convection mode, a mixed film boiling and

natural convection mode, and a pure film boiling mode.”  However, although those wires are three to ten times the diameter of the 7.5 micron platinum wires of this work; this work clearly revealed nucleate boiling from the small wires. Balhru and Lienhard only collected data at low pressures; it would be a relatively easy experiment to deploy those wires at higher pressures in order to reveal a transition to nucleate boiling.

referenceS

1. Bakhru, N. and Lienhaard, J. H., Boiling from small cylinders, Int. J. Heat and Mass Transfer, vol. 15, pp. 2011-2025, 1972.

2. Leyse, R. H., Method for monitoring for the presence of dissolved gas in a fluid under pressure, United States Patent 5,621,161 April 15, 1997.

3. Leyse, R. H., Microscale heat transfer to pressurized water at ultra-high power density, Proceedings of the International Conference, Thermal Challenges in Next Generation Electronic Systems, Millpress, pp195-197, 2002.