Study Of Cooling Heat Transfer To Methane And Methane/Nitrogen Mixtures Under Supercritical Pressure

Coalbed methane （CBM） is a kind of potential valuable energyresource. In China, CBM is usually mixed with air during the coal miningprocess. After removing O2, CO₂and H2S etc. by a series ofpre-purification treatments, CBM is mainly consisted of methane andnitrogen. Liquefaction of CBM is a good approach for its utilization.Generally, the CBM liquefaction is conducted under supercritical pressure.Therefore, cooling of supercritical methane and methane/nitrogenmixtures is one of the crucial parts during the CBM liquefaction. In thepresent study, cooling heat transfer to methane and methane/nitrogenmixtures is experimentally, theoretically and numerically investigatedunder supercritical pressure. The following conclusions are achieved:（1） Density, specific heat capacity at constant pressure, viscosity andthermal conductivity of methane/nitrogen mixtures are deduced andcalculated. The binary interaction parameters kijof PR equation-of-state（EOS） are extrapolated for calculating density and specific heat capacityat constant pressure, and the correlations of kijare developed. Theviscosity model based on PR EOS is more accurate than that based onlaw of corresponding states. The thermal conductivity model based onlaw of corresponding states predicts well.（2） Cooling of supercritical methane and methane/nitrogen mixturesin a horizontal tube are experimentally investigated. The study mainlyfocuses on the effects of mass flux, inlet pressure and nitrogen content onheat transfer characteristics of supercritical methane. The results showthat the heat transfer coefficient gradually grows with decreasing bulktemperature, and reaches its maximum at the pseudo-critical point, thendrops as bulk temperature further decreases. For a given pressure, an increase in mass flux corresponds to an increase in heat transfercoefficient. For a given mass flux, the heat transfer coefficient istransferred to higher temperature with the increase of inlet pressure, andthe maximum decrease. For given mass flux and inlet pressure, the heattransfer coefficient is transferred to lower temperature with the increaseof nitrogen content, and the maximum decrease. The experimental resultsof nitrogen content effect also indicate that the heat transfer tomethane/nitrogen mixtures under supercritical pressure is stronglyaffected by the properties in the vicinity of pseudo-critical point.（3） The correlations of supercritical fluids cooling in open literatureare reviewed and compared with the previous experimental data. Theresults show that when the heat flux is low, the buoyancy effect is weak,and most of the correlations agree well with the experimental data; whenthe heat flux is high, the buoyancy effect becomes stronger, and almost allthe correlations fail to predict the heat transfer coefficient. Combinedwith the previous conclusions, the influencing factors are summarizedand changed to the equation with dimensionless numbers. By fitting theexperimental data of methane and methane/nitrogen mixtures, thecorrelations are developed.（4） Cooling of CO₂, methane and methane/nitrogen mixtures arenumerically investigated in horizontal and vertical tubes undersupercritical pressure. The results show that LB low Reynolds turbulencemodel can be qualified for simulating the supercritical convective heattransfer.In the study of supercritical CO₂cooled in a horizontal tube, forcedconvection is the prime heat transfer mechanism whenGr/Re²0.01;the mixed convection of both forced and free convections dominate inturbulence flow and the heat transfer is enhanced during supercritical CO₂cooling when0.01<Gr/Re²0.1; free convection is predominant andforced convection is negligible whenGr/Re²0.1.In the study of supercritical CO₂cooled in a vertical tube, buoyancyeffect enhances the heat transfer in upward flow, and impairs the heattransfer in downward flow. The velocity profile in upward flow isparabolic distribution, and it becomes inverted in downward flow because of the buoyancy effect.In the study of supercritical methane cooled in a horizontal tube,buoyancy effect makes the secondary flow generate in the cross-section,therefore enhancing the heat transfer in top wall surface of the horizontaltube and impairing the heat transfer in bottom wall surface. Thisphenomenon is similar to the heat transfer feature in vertical tubes. Theheat transfer in top wall surface in horizontal tubes is equivalent tobuoyancy-opposed mixed convection in vertical tubes, the heat transfer inbottom wall surface to buoyancy-aided mixed convection. Among fourcriteria for evaluating the buoyancy effect on heat transfer, the expressionGr/Re^2.710^-5andGr/Re²10^-2are relatively better than the othertwo.In the study of supercritical methane cooled in a vertical tube, whenthe buoyancy effect is considerably strong, the fluid flow from the nearwall region to the core in cross-section in upward flow, and thisstrengthens the turbulence intensity and enhances the heat transfer.However, it is contrary in downward flow. When the buoyancy effect isnegligible, the heat transfer coefficient in horizontal and vertical tubes isalmost identical with each other, and the gravity effect is also neglected.The predicted results with LB model agree well with Gnielinskicorrelation.The effects of heat flux, nitrogen content and inlet pressure on heattransfer indicate again that the supercritical convective heat transfer isstrongly affected by the properties in the vicinity of pseudo-critical point.