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Heat Transfer in Helium Injected Liquid Nitrogen

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Heat Transfer in Helium Injected Liquid Nitrogen Heat Transfer In Helium Injected Liquid Nitrogen Fenner Colson1,2, Dogan Celik2,3, Steven W. Van Sciver2,3 1Department of Physics and Astronomy, Minnesota State University College of Social and Natural Sciences, Moorhead, MN 56563 USA 2National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA 3Mechanical Engineering Department, FAMU-FSU College of Engineering, Tallahassee, FL 32310, USA ABSTRACT RESULTS AND CONCLUSIONS Define the coefficients: ρ = liquid density L cLΔT Liquid nitrogen boiling suppression is a known phenomenon occurring when Ja = = Jakob number ρG = vapor density The change in temperature due to heat flux through the aluminum disk was measured i gaseous helium is injected directly into boiling nitrogen. The heat transfer coefficient, fg µL = liquid viscosity for a number of different power inputs. The ∆T is illustrated in the graph below. c = liquid specific heat which determines how efficiently heat is transmitted from a heat source to a material, is not p L SAT g = gravitational acceleration the same in boiling liquid nitrogen and helium injected liquid nitrogen. This change is not Helium Injected at 17 Watts KP = 1/ 2 85 [gσ L (ρL − ρG )] Pr = liquid Prandtl no. due to the temperature drop of the nitrogen, nor from the chemical interaction of helium Bath Temperature L Heater Temperature σL = surface tension gas and liquid nitrogen, but because of some other mechanism not covered by the scope 83 i = heat of vaporization. of this project. Heater fg turned on. 81 Evaluation of the above equation with the appropriate values yields the power INTRODUCTION transfer per unit area as a function of ∆T. Using those values with Newton’s 79 ∆T € Law of Cooling produces a set of heat transfer coefficients that we can Helium Suppression of boiling in liquid nitrogen is valuable in experiments where the (K) Tempertature injected. compare to the helium injected data. optical or vibrational disturbances should be minimized. Once the helium is injected into 77 the liquid nitrogen, the nitrogen drops several degrees. Thus the injection of helium 2 75 Heat Transfer Coefficient (W/m !K) causes the liquid nitrogen to cool down beyond its boiling point, which then allows absorbed heat to increases nitrogen temperature, rather than changing the state. ∆T = 6.51 K ∆T = 8.08 K ∆T = 9.51 K 73 0 500 1000 1500 2000 2500 3000 As the helium is injected, the liquid nitrogen evaporates directly into the helium Time (s) He injected 848.54 2906.34 9085.27 bubbles [1],[2]. Evaporation of the nitrogen is the mechanism by which the temperature Boiling correlation 855.79 804.00 672.06 decreases. Figure 3: Data showing temperature difference due to heater and helium injection. The method of extracting ∆T, as demonstrated above, was replicated for all data runs ∆T = 2.43 K ∆T = 3.84 K ∆T = 4.99 K ∆T = 5.75 K ∆T = 6.53 K with plain liquid nitrogen as well as helium injected. Power input per unit area is known since Liquid Nitrogen 3452.32 7247.40 10079.92 12040.57 13895.17 the disk was held at a measured voltage. Heat Transfer Rates Figure 6: Summary of heat transfer coefficients. The helium and nitrogen 10 values are calculated by direct division of data from Figure 4. 9 8 Figure 4: Summary of data The heat transfer coefficient for helium injected liquid nitrogen agrees 7 analysis. Power inputs are nicely with the equivalent super-cooled nitrogen at low ∆T, but as ∆T 6 plotted as a function of ∆T increases, the difference rises dramatically. Thus ∆T cannot be the only factor 5 Liquid Nitrogen so the heat transfer in changing the heat transfer coefficient. 4 LN2 with Helium Figure 1:Boiling liquid nitrogen and result of helium injected into liquid nitrogen [3] . (W/cm^2) Q/A coefficient can be evaluated Figure 7: 3 Heluim Injected Hydrogen Injected 85 directly from the data. 84 Comparison of 2 83 82 This reaction has the potential to change certain physical properties, let’s consider 81 80 1 helium and 79 78 Newton’s Law of Cooling: 77 76 0 hydrogen injected Temperature (K) 75 Temperature (K) 74 0 1 2 3 4 5 6 7 8 9 10 73 72 Q ∆T (K) -100 400 900 1400 1900 2400 2900 3400 0 500 1000 1500 2000 into nitrogen. = hΔT [4]. Time (s) Time (s) A Our area of interest lies in the ∆T values, which define how the heat transfer coefficient is Calculations from Figure 7 reveal that the gas itself has no effect on Our interest lies in the proportionality constant h (W/m2K), the heat transfer coefficient, behaving. Evaluating the quotient of Q/A and ∆T at a point will be the means that the heat the heat transfer coefficient, since the results are the same if helium or and how it behaves under the condition of helium injection. transfer coefficient is calculated. The accepted format to present the data is on a logarithmic hydrogen is used. Therefore the reason for the coefficient change cannot be scale graph. related to molecular interactions with the gas, nor can the coefficient change €EXPERIMENTAL METHODS Heat Transfer Rates (Log Scale) be explained by ∆T. One possibility is that film boiling, which occurs at higher 100 ∆T, is affected by the helium injection. This interaction may reduce film Our measurement apparatus consisted of a bulb boiling, which would normally increase the heat transfer coefficient. shaped glass dewar into which we suspended a rod to Implications of this include helium injected nitrogen being used as a coolant, which we attached an aluminum disk. This aluminum disk Figure 5: Logarithmic since its ability to transfer heat is improved. The scope of the project did not 10 is surrounded by G-10 insulation, with one face being open scale of Figure 4 with investigate this avenue, but future exploration should consider that possibility. experimental nucleate/ to the liquid nitrogen, and the other face having a resistive Liquid Nitrogen heater and insulation. Two thermocouples were run down (W/cm^2) Q/A LN2 with Helium film boiling for nitrogen superimposed. REFERENCES the rod to monitor the temperature of the nitrogen bath and 1 the temperature of the disk. 0.01 0.1 1 10 100 1000 Helium and hydrogen were both used in the [1] S. Takayoshi, W. Kokuyama, H. Fukyama The Boiling Suppresion of Liquid research, and were supplied to the liquid nitrogen by direct Nitrogen Cryogenics 49 (2009) pg 221 injection from a metal tube. A flow meter was connected in 0.1 [2] G. Minkoff, F. Scherber, A. Stober Suppression of Bubbling in Boiling ∆T (K) series to the hose. A constant flow was applied to assure Refrigerants Nature Vol. 180 pg 1414 that the amount of gas being injected was not a factor in Figure 2: From Figure 5 we can conclude that the heat transfer coefficient does change between liquid [3] S. Takayoshi, et. al. pg 222 th the experiments. Experimental setup. nitrogen and helium injected liquid nitrogen since there is a horizontal shift from left to right. [4] F. Incropera and D. DeWitt Fundamentals of Heat and Mass Transfer 4 The heat transfer coefficient of liquid nitrogen was experimentally verified first. The This fact, however, is not necessarily a shocking discovery, because we expect a change of Ed. © 1996 John Wiley & Sons pg 8 aluminum disk was subject to a certain amount of heat, which allowed it to remain at a coefficient due to the change of ∆T. Comparing the equivalent super-cooled liquid nitrogen [5] R. Barron Cryogenic Heat Transfer © 1999 Taylor and Francis pgs 164-166 different temperature than the nitrogen bath. By observing the behavior of the heat transfer coefficient to observed helium injected coefficient will reveal reason for temperature of both the disk and nitrogen, the heat transfer coefficient can be calculated coefficient change. For this purpose we can use the boiling correlation ACKNOWLEDGEMENTS using Newton’s Law of Cooling. The coefficient for liquid nitrogen can be compared with 0.3 Special thanks to Dr. Dogan Celik, Dr. Steven W. Van Sciver, and the entire % ( % (0.7 known values to verify the quality of experiment. The same process was applied to helium Ja (Q/ A) σ L ρG Cryogenics Laboratory Group for their guidance and assistance in running the = 0.0007' * ' KP * [5]. injected nitrogen, and the heat transfer coefficient for liquid nitrogen at the equivalent (Pr )0.65 µ i g(ρ − ρ ) ρ experiment; Mr. Jose Sanchez, and the staff of the National High Magnetic super-cooled temperature was used for comparison. L & L fg L G ) & L ) Field Laboratory Center for Integrated Research and Learning; Work funded by the NSF Cooperative Agreement DMR-0654118, NSF DMR-0645408, Florida State University € .
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