Research PaperInvestigations of heat and momentum transfer in two-phase injector operating with isobutane
Introduction
Low or medium temperature heat sources can be used as a motive heat in thermally driven refrigeration systems: ejection systems and sorption systems (absorption and adsorption ones). Depending on their availability solar, geothermal and other renewable or waste heat sources may be applied for this purpose. However, in all of these systems liquid pump is the most problematic component.
Two-phase vapour-liquid injectors are driven by heat and this feature makes these devices as potential alternative to mechanical classic liquid pumps. The possible application of a two-phase injector instead of a mechanical pump for the ejection refrigeration system is presented in Fig. 1. One of the advantages of the presented systems is lack of electric power needed to drive the system because the injector is driven by the same vapour as the vapour ejector. However, the injector requires only a small part of the vapour produced in the vapour generator. This feature makes that this system becomes “a green system” driven by heat only. Another important feature of the injector is its simple design in comparison with mechanical pump. Injector does not require lubricants and has no moving parts.
On the basis of the previous analyses of the operation of the injector in refrigeration systems [1], [2] it may be concluded that required power consumption to pump the liquid is relatively small, less than few percents in comparison to the motive heat. One of the disadvantages of the injector is its relatively small efficiency. However, when the system is driven by waste heat or renewable energy source, the moderate efficiency may not be considered as important, due to very low cost of the heat source. Also, low efficiency of the injector may not produce negative effect on the efficiency of the ejection refrigeration system. Therefore the use of the vapour-liquid injector as a substitute for the mechanical pump leads to slight increase in demand for the motive heat, as the motive vapour produced in the vapour generator must drive both: the refrigeration ejector and vapour-liquid injector.
Research investigations of the vapour-liquid injector applications are very limited. Only few reports dealing with possible application of vapour-liquid injector in refrigeration systems can be found [1], [2], [3], [4]. In most cases steam-water injectors were taken into consideration where injectors are applied as a passive cooling system for nuclear power plants [5], [6], [7], [8], [9]. Xing et al. [10] and Smierciew et al. [11] theoretically investigated the possible application of two-phase injector as a liquid compression device for refrigeration system. Refrigerants R290 and R404A in [10], and isobutane R600a in [11] were selected for these theoretical analyses. Previous investigations of steam-water injector [12], [13], [14], [15] revealed that for lower liquid flow rates the discharge pressure diminishes but higher liquid temperature rise could be gained which makes the injector an effective heat exchanger. It was reported [15] that for the steam-water injectors, average heat transfer coefficient may approach 1 MW/(m2 × K). Heat transfer coefficient at this level is two order of magnitude higher than in classic heat exchangers used in steam power plants. Recently operation of two-phase vapour liquid injectors for various applications was experimentally investigated [5], [16] for the case of water substance only, however.
Unfortunately no experimental investigations dedicated to two-phase injectors for refrigeration applications are available in literature. Heat transfer rate inside the two-phase injectors was preliminary investigated experimentally by the present authors for the case of vapour-liquid injectors operating with isobutane as a working fluid [17], [18]. The present paper present the results of the systematic experimental investigations of the two-phase injector for isobutane as a working fluid.
The efficiency of the two-phase ejector components were investigated for the case of isobutane as working fluid [19]. The dimensionless correlations were proposed for the component efficiencies. The pressure ratio was selected as the quantity that influences the component efficiencies of the two-phase ejector. However, no such investigations were reported before for the case of the two-phase injector. Moreover, because of complicated flow structure inside the injector it may be expected that the component efficiencies should depend on various quantities that describe two-phase flow inside the injector.
It is a clear need for the experimental data showing operation of two-phase vapour-liquid injector in refrigeration applications. In this paper the component efficiencies were investigated along with condensation heat transfer inside the mixing chamber. Because of complicated nature of the two-phase flow that is formed in the injector the components efficiencies should not be treated as constant quantities since they should depend on the two-phase flow features, i.e. two-phase flow pattern inside the injector components. This approach was proposed in the present paper. On the basis of the previous studies [11] isobutane was chosen as as working fluid as a promising and perspective low GWP fluid.
Section snippets
Operation of the two-phase injector
The geometrical configuration of the two-phase vapour-liquid injector is presented in Fig. 2. The primary (motive) fluid for two-phase injector is vapour at temperature depending on the heat source temperature and pressure corresponding to the saturation condition. Secondary fluid is the subcooled liquid that flows from the condenser of the refrigeration system. Primary fluid undergoes expansion to low pressure and acceleration in the motive nozzle. Since the injector operates under conditions
Experimental apparatus and procedure
The schematic of the experimental testing stand dedicated for the investigations of the two-phase injectors is presented in Fig. 3 and tested injector is presented in Fig. 4a, Fig. 4b. Main dimensions of the tested ejector are given in Table 1.
Motive nozzle position can be changed within the range 0–0.50 mm, where 0 mm means no liquid flow, since nozzle touches the mixing chamber surface. Therefore the cross-section area for the entrained liquid is variable, and the thickness of the liquid gap
Experimental results of injector operation
Experimental investigations covered measurements for the motive nozzle of the throat diameter 1.50 mm with position of the nozzle changed so that three various thickness of the liquid gap (liquid throat) were applied, namely: δ1 = 0.13 mm, δ2 = 0.21 mm, and δ3 = 0.30 mm. The control valve 9 (see Fig. 3) was used for control of the discharge pressure at the injector outlet. As an effect, the mass entrainment ratio was changed due to changes of the discharge pressure. During experiments the mass
Motive nozzle
Motive nozzle in the vapour-liquid injector is the converging-diverging de Laval nozzle since the injector operates under conditions of suction to motive pressure ratio that is lower than critical pressure ratio. It is very common that this type of well performed nozzle for single phase vapour flow has efficiency higher than 0.90. The relationship between nozzle efficiency and velocity coefficient is , therefore, the velocity coefficient has almost constant value . Because of this
Conclusions
On the basis of presented results the following conclusion may be drawn:
- 1.
The operation of the two-phase vapour-liquid injector for isobutane as a working fluid was tested systematically. To the knowledge of the authors these results are the first ones in literature for the two-phase vapour-liquid injector operating with refrigerant as a working fluid.
- 2.
Although the compression efficiency of the tested injector is less than 1%, the total efficiency of the injector for was less than 28% for
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