| 1. |
- Ignatowicz, Monika, 1983-, et al.
(författare)
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ISO-PARAFFINS AS LOW TEMPERATURE SECONDARY FLUIDS
- 2017
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Ingår i: 5TH IIR INTERNATIONAL CONFERENCE ON THERMOPHYSICAL PROPERTIES AND TRANSFER PROCESSES OF REFRIGERANTS (TPTPR). - : INT INST REFRIGERATION. ; , s. 436-442
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Konferensbidrag (refereegranskat)abstract
- Ethylene and propylene glycol water based secondary solutions are very popular due to their low corrosivity character, low freezing point, high boiling point and low fire hazards. The main drawback of ethylene glycol solutions is its potential toxicity both to the environment, animals and food products. Therefore, propylene glycol is the most common substitute for ethylene glycol. The biggest disadvantage of propylene glycol solutions is its very high viscosity at low temperatures, which results in higher pumping power and lower system performance. For very low temperatures, it is common to use non-aqueous synthetic fluids, which have lower viscosities at these temperatures. Thus, the aim of this paper is to propose a new generation of synthetic iso-paraffins having better thermophysical properties compared to existing synthetic products used for very low temperature applications. The first results showed that the new generation of iso-paraffins having the freezing temperature of - 50 degrees C or lower had significantly lower viscosity (between 75 % and 98 %) at very low temperatures compared to 60 wt-% propylene glycol (PG 60 %) and 60 wt-% ethylene glycol solution (EG 60 %). Iso-paraffin 1 showed better performance than iso-paraffin 2 by giving the lowest freezing point, the lowest dynamic viscosity and kinematic viscosity as well as the highest thermal conductivity. Instead, iso-paraffin 2 had higher specific heat capacity. As seen, the different composition of iso-paraffin mixture can result in different properties and operating temperature ranges. Thus, iso-paraffins could become an interesting alternative for very low temperature cooling applications.
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| 2. |
- Anwar, Zahid, et al.
(författare)
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Flow Boiling Heat Transfer and Dryout Characteristics of R600a in a Vertical Minichannel
- 2015
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Ingår i: Heat Transfer Engineering. - : Informa UK Limited. - 0145-7632 .- 1521-0537. ; 36:14-15, s. 1230-1240
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Tidskriftsartikel (refereegranskat)abstract
- Refrigerant-related environmental concerns forced legislative bodies to phase out some types of refrigerants, namely, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and in the near future European legislation will be affecting hydrofluorocarbons (HFCs) as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental findings on flow boiling heat transfer and dryout characteristics of isobutane (R600a) in a uniformly heated, vertical, stainless-steel test section (1.60mm inside diameter and 245mm heated length) are reported in this article. The experiments were conducted at two saturation pressures corresponding to the temperatures of 27 and 32 degrees C, with five mass fluxes in the range 50-350kg/m(2)-s and at outlet vapor qualities up to dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The dryout heat flux increased with increasing mass flux; however, no significant effect of varying saturation temperature was observed. The experimental results (for heat transfer and dryout) were compared with different macro and microscale correlations from the literature.
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| 3. |
- Björk, Erik, et al.
(författare)
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A thermographic study of the on-off behavior of an all-refrigerator
- 2010
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 30:14-15, s. 1974-1984
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Tidskriftsartikel (refereegranskat)abstract
- In this work a thermographic camera is used to observe the temperature distribution of a household refrigerator cooling system operating at on off cycling conditions. This technique offers an alternative method to analyze the cooling system compared to conventional thermocouples. In particular it is interesting to view the overall picture of how the refrigerant charge is distributed over the cooling system at transient conditions. In addition, four sources of energy losses were identified and discussed. Out of these losses, two would have been difficult to find using conventional thermocouple temperature measurements. (C) 2010 Elsevier Ltd. All rights reserved.
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| 4. |
- Claesson, Joachim, 1970-
(författare)
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Thermal and hydraulic performance of compact brazed plate heat exchangers operating as evaporators in domestic heat pumps
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates the performance of compact brazed plate heat exchangers (CBE) operating as evaporator in heat pump applications. The thesis, and the performances investigated, has been divided into three main sections; One zone evaporator performance; Two zone evaporator performance; and finally Local performance. The 'One zone evaporator performance' section considers the evaporator as one "black box". It was found that "approaching terminal temperatures" were obtained as low overall heat flux is employed. It was also found that the total area averaged film heat transfer coefficient was affected by changes of the brine mass flow rate. This indicates that the widespread Wilson plot method may not be used to determine flow boiling heat transfer coefficients. Further, it seems that co- and counter-current flow configuration performs equally well if the superheat is kept low. A numerical simulation of the above investigations indicates that a nucleate boiling model better predicts the performance compared to a convective evaporation model. Finally, the significant impact of the refrigerant inlet distributor design was illustrated using several CBEs with different inlet geometries but with identical heat transfer surfaces. The 'Two zone evaporator performance section' considers the evaporator as two "black boxes", i.e. the boiling and superheating boxes. Thermochromic liquid crystals (TLC) was used to determine the boiling heat transfer area. The resulting flow boiling heat transfer coefficient was found to correlate with heat flux. The superheated heat transfer area was then estimated using single phase correlations. It was observed that the TLC measurements and the predicted superheating area did not agree. Possible causes for this deviation were discussed. The most likely explanation found was the presence of mist flow at the higher vapor quality range in the boiling section of the evaporator. The 'Local Performance' section considers local pressure drop and flow boiling heat transfer. The Chisholm parameter was found not to be a constant and was found to correlate well with the kinetic energy per volume. The resulting predictions of the pressure drop were better than ± 10%. The resulting local flow boiling heat transfer coefficient, at different vapor quality, mass flux and heat flux, was compared to flow boiling correlations available in the literature. It was found that the saturated nucleate pool boiling correlation by Cooper (1984) and narrow channel flow boiling correlations (Tran 1999, Lazarek and Black 1982) predicted the experimental data better than several traditional flow boiling correlations, developed for larger tubes.
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| 5. |
- Grozdek, Marino, et al.
(författare)
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Experimental investigation of ice slurry heat transfer in horizontal tube
- 2009
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Ingår i: International journal of refrigeration. - : Elsevier BV. - 0140-7007 .- 1879-2081. ; 32:6, s. 1310-1322
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Tidskriftsartikel (refereegranskat)abstract
- Heat transfer of ice slurry flow based on ethanol-water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 degrees C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus-Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. it was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus-Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re-cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results. (C) 2009 Elsevier Ltd and IIR. All rights reserved.
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| 6. |
- Ignatowicz, Monika, 1983-, et al.
(författare)
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Alternative alcohol blends as secondary fluids for ground source heat pumps
- 2016
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Ingår i: Refrigeration Science and Technology. - : International Institute of Refrigeration. - 9782362150180 ; , s. 610-617
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Konferensbidrag (refereegranskat)abstract
- The most common secondary fluid used for the borehole heat exchangers in Sweden is an aqueous solution of ethyl alcohol (EA) due to its relatively good thermophysical properties and low toxicity. Commercially available ethyl alcohol based fluids in Sweden contain up to 10 wt-% denaturing agents in form of propyl alcohol (PA) and n-butyl alcohol (BA). The aim of this paper was to investigate the performance of the existing ethyl alcohol blend containing two denaturing agents and alternative alcohol blends in terms of the pressure drop and heat transfer in the BHE and comparison with ethyl alcohol based secondary fluid. Experimental results showed that the presence of these denaturing agents improves thermophysical properties such as specific heat capacity, thermal conductivity and dynamic viscosity when added in small concentration. EA18 + PA1.6 + BA0.4 and EA18.4 + PA1.6 present the best characteristics in terms of the heat transfer and pressure drop. Both blends are giving higher heat transfer coefficient by 9.4 % (EA18 + PA1.6 + BA0.4) and 8.11 % (EA18.4 + PA1.6) than pure EA20. Both blends are giving as well lower pressure drop than EA20 by up to 2.7 % (EA18 + PA1.6 + BA0.4) and 3 % (EA18.4 + PA1.6). EA18 + PA1.6 + BA0.4 gives 1.4 % higher heat transfer coefficient and EA18.4 + PA1.6 gives lower pressure drop by up to 0.4 % when these two blends are compared.
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| 7. |
- Ignatowicz, Monika, et al.
(författare)
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Ethyl and isopropyl alcohol blends as alternative secondary fluids
- 2015
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Ingår i: Refrigeration Science and Technology. - : International Institute of Refrigeration. - 9782362150128 ; , s. 2224-2231
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Konferensbidrag (refereegranskat)abstract
- Methyl and ethyl alcohol based secondary fluids are commonly used in different indirect refrigeration systems due to relatively good thermophysical properties. Methyl alcohol is considered toxic and not permitted for use as secondary fluid in most of Europe. Ethyl alcohol based secondary fluids are commonly used in Sweden due to their low toxicity and good properties. Commercially available ethyl alcohol based fluids contain up to 10 wt-% denaturing agents in form of propyl alcohol (2-propanol, isopropanol) and nbutyl alcohol (n-butanol). Results show that presence of propyl alcohol in the ethyl alcohol based secondary fluid improves its thermophysical properties such as specific heat capacity, thermal conductivity and dynamic viscosity when added in small concentration. The aim of this paper is to investigate the thermophysical properties of various ethyl and isopropyl alcohol blends in order to find the most optimal blend as well as to evaluate if this blend could become a competitive alternative secondary fluid.
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| 8. |
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| 9. |
- Liu, Qingming
(författare)
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Numerical study of flow boiling in micro/mini channels
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Boiling phenomena in micro scale has emerged as an interesting topic due to its complexity and increasing usage in micro electronic and mechanical systems (MEMS). Experimental visualization has discovered five main flow regimes: nucleate boiling, isolated bubbles, confine bubbly flow, elongated bubbly (or slug) flow, and annular flow. Two of these patterns (confine bubbles and slug flow) are rarely found in macro channels and are believed to have very different heat transfer mechanisms to that of nucleate boiling.The development of a phenomenological model demands a deep understanding of each flow regime as well as the transition process between them. While studies in every individual flow pattern are available in literature, the mechanisms of transition processes between them remain mysterious. More specifically, how the isolated bubbles evolve into a confined bubbly flow, and how this further evolves into elongated bubbles and finally an annular flow. The effects of boundary conditions such as wall heat flux, surface tension, and interfacial velocity are unclear, too.The aims of this thesis are to develop and validate a new numerical algorithm, perform a comprehensive numerical study on these transition processes, uncover the transition mechanisms and investigate effects of boundary and operating conditions.Firstly, a sophisticated and robust numerical model is developed by combining a coupled level set method (CLSVOF) and a non-equilibrium phase change model, which enables an accurate capture of the two-phase interface, as well as the interface temperature.Secondly, several flow regime transitions are studied in this thesis: nucleate bubbles to confined bubbly flow, multi confined bubbles moving consecutively in a micro channel, and slug to annular flow transition. Effects of surface tension, heat flux, mass flux, and fluid properties are examined. All these regimes are studied separately, which means an appropriate initial condition is needed for each regime. The author developed a simplified model based on energy balance to set the initial and boundary conditions.
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| 10. |
- Palm, Björn
(författare)
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Hydrocarbons as refrigerants in small heat pump and refrigeration systems - A review
- 2008
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Ingår i: International journal of refrigeration. - : Elsevier BV. - 0140-7007 .- 1879-2081. ; 31:4, s. 552-563
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Forskningsöversikt (refereegranskat)abstract
- Due to the concern for the effects of the release of HFC refrigerants on the global environment caused by the high global warming potential of these substances, there is a large interest in Europe and elsewhere for the use of hydrocarbons as refrigerants. This article presents a comparison of the properties and performance of hydrocarbons as refrigerants in small-size heat pump and refrigeration systems (< 20 kW cooling). A listing of several commercially available systems is also presented. The designs, safety precautions and performances of some of these systems are described. As a general conclusion, it is shown that using hydrocarbons will result in COPS equal to, or higher than, those of similar HFC systems. It is also shown that components suitable for hydrocarbon systems are available on the market, even though the number of large-size hermetic compressors is limited. A major concern, which should not be taken lightly, is the safety issue. Reduced charge through indirect systems and compact heat exchangers, outdoor placing of the unit, hydrocarbon sensors and alarms and forced ventilation are all steps which may be applied to reduce the risks under normal operation.
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