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- Berntsson, Thore, 1947, et al.
(författare)
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Heat transfer of nonazeotropic mixtures in a falling film evaporator
- 1985
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Ingår i: ASHRAE:s Annual Meeting, Honolulu, 23-27 June 1985: - ASHRAE Transactions. ; 91:1, s. 1337-1350
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The evaporative heat transfer coefficient for nonazeotropicmixtures of R12 and R114 has been studied in a falling filmheat exchanger. The main aim with the project has been tostudy how the heat transfer coefficient is influenced bythe additional mass transfer resistance that is introducedwhen a mixture is used instead of a pure refrigerant.The test apparatus consisted of one tube, which was heatedfrom the outside and with the refrigerant forming a fallingfilm on the inside. The heating was done by a liquidflowing in an annular space countercurrently to the fallingfilm.From the results it can be concluded that the heat transfercoefficients for the mixtures at surface evaporation conditions are between the corresponding ones for the pure refrigerants at given Re numbers. This indicates that theadditional mass transfer resistance when a mixture is usedis small in a falling film at such conditions. This isprobably due to the small film thickness.The heat transfer coefficients versus the heat flux werealso measured. Again, the values for the mixtures at agiven'heat flux fall between the corresponding for thetwo pure refrigerants, both at surface evaporation andnucleate boiling conditions.
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| 2. |
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| 3. |
- Munch, Karin, 1954, et al.
(författare)
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Large Heat Pumps Nonazeotropic mixtures as working Fluids - some results from computer simulations
- 1985
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Ingår i: IIR Conference "Comission E2 - Heat Pumps and Energy Recovery, Trondheim, 19-21 June, 1985.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Nonazeotropic mixtures of working fluids have been much discussed during recent years for large heat pumps, although practically no systems have been really installed so far. Hith mixtures it is possible to achieve gliding temperatures in the condenser and the evaporator. This enables several possibilities for improvement of the performance, such as the COP, the heating capacity, variation of the heating capacity with the load, pressure levels. and compressor discharge temperatures.In the following comparisons between some mixtures and pure working fluids regarding the COP and the relative heating capacity are presented. The results shown are computer calculated ones.Most calculation results for mixtures. presented so far in the literature, are valid for a heat pump cycle with infinite heat exchanger areas. In this paper it is shown that the influence on the performance of finite heat exchanger areas and realistic compressor efficiencies is considerable.
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| 4. |
- Munch, Karin, 1954
(författare)
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Nonazeotropic mixtures as working fluids in large heat pumps - Heat transfer in a falling film evaporator and system simulations
- 1986
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis is divided into two main parts, the first onedealing with heat transfer in the evaporator and the secondone dealing with simulations of the whole heat pump cyclewith nonazeotropic mixtures. One of the aims with the heat transfer studies was to investigate if the additional mass transfer resistance, introduced when mixtures are used, can be reduced in a thin falling film with high turbulence compared with corresponding one for a horizontal tube. The measurements were carried out in one smooth test tube with the mixtures on the inside. The investigated fluids are R22/R114, R12/R114 and R12/R11 and the corresponding pure fluids. For the mixtures the transition point between surface evaporation and nucleate boilng was depressed compared to pure fluids. This was found to be the main reason for the decrease of the heat transfer coefficients for mixtures. When all fluids were in the surface evaporating regime this decrease was small, as a maximum 21 %. The magnitude of the heat transfer coefficients varied between approximately 950 W/m2K and 4 000 W/m2K depending on Re number, heat flux and type of fluid.With aid of the computer program a parameter study on heatpump systems with nonazeotropic mixtures has been performedin the second part of this work. This study has been carried out for three case studies, two industrial ones and one space heating case. The results are shown as COP, heating capacity and condensing pressure. A special emphasis has been laid on the influence on these parameters of the heat exchangers size and a comparison between horizontal and falling film evaporators. The calculations have shown that, for given heat exchangers, there are in some cases possibilities for considerable increases of the COP. In favourable situations this improvement is in the order of 15-20 %. The heating capacity can be increased considerably in all cases where the "mean volatility" of the mixture is higher than that of the pure fluids, with which the comparison is made. Improvements of more than 100 % are possible to achieve. The comparison between falling film and horizontal evaporators has shown that the COP for a given area in some cases can be increased when the falling film type is used. In many cases the same COP can be achieved with a considerably smaller evaporator size, 50 % or less in the falling film case. This fact should make falling film evaporators economically interesting competitors to horizontal heat exchangers in the future.
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