1 - Acta Technica Corviniensis

ACTA TECHNICA CORVINIENSIS – Bulletin of Engineeringof Dittus‐Boelter single‐phase heat transfercorrelation. The two‐phase model the reducedpressure refrigerant takes into account.0.76 0.04⎡0.8 3.8⋅x⋅( )( 1−x)⎤α kf = α f ⋅⎢1−x +*0. 38 ⎥ (15)⎣p ⎦Where the reduced pressure is:∗p =pp critIn Eqs. (11) the single phase heat transfer coefficientsare determined by the Dittus‐Boelter correlation.Compressor modelRefrigerant mass flow rate through the compressor isa function of compression ratio, refrigerant densityand compressor speed, that is,. ⎛ p ⎞⎜cm = f , ρ sz , N⎟(16)⎝ pe⎠The relationship between compressor exit and inlettemperatures is given by:k−1⎡ ⎤⎢⎛p ⎞ knyT⎥ny = Tsz⋅⎢⎜⎟ − 1⎥(17)⎢⎝psz⎠⎣ ⎥⎦Neglecting the thermal inertia effects, the indicatedwork is given by:k−1⎡ ⎤k ⎢⎛p ⎞ knyW = ⋅p⎥sz ⋅vsz⋅⎢⎜⎟ − 1k − 1⎥(18)⎢⎝psz⎠⎣ ⎥⎦Isentropic effiencies [10] is correlated as a function ofthe pressure ratio between condensation pressureand evaporation pressure.2η = A +⋅B⋅τ+ C ⋅τ(19)where A, B and C are the regression coefficients.THERMOSTATIC EXPANSION VALVE MODEL AND REFRIGERANTThermostatic expansion valve is the valve thatcontrols the refrigerants mass flow rate by sensing thedegree of suction vapor superheat temperature. Theenthalpy is assumed to be constant. The refrigerantmass flow is calculated by the following equation.( p − p )m = C ⋅ 2ρ ⋅(20)where C is the characteristic constant of the valve.The refrigerants used in this study are the R22, R134a,R407C, R410A. Calculation of the refrigerants andtransport properties is performed by correlationswritten as computer code function, usingthermodynamic properties coming from SOLKANEdatabase [11]. The effect of the circulation of oil is nottaken into account in the model.INITIAL CONDITION AND VALUESThe mathematical models are simulated by the use ofthe software tool Solkane.The initial conditions and values for the simulation: Refrigerant: R22, R134a, R407C, R410A Refrigerating capacity: Q = 2kWo Temperature of evaporator: T o = 0 C Pressure drop in evaporator: Δ p =0.4bar Superheating temperature: Δ T = 5Kce Temperature of condenser: T c = 45 C Pressure drop in condenser: Δ p = 0.1bar Isentropic efficiency of compressor: η =0. 8SIMULATION RESULT AND DISCUSSIONRefrigerants:Figure 2. The coefficient of performance of the refrigeratorRefrigerantsFigure 3. Performance of the compressorRefrigerants:Figure 4. Specific latent heat of the refrigerantsRefrigerants:Figure 5. Volume capacity of the refrigerantsRefrigerants:Figure 6.Volume flow of the refrigerantsRefrigerants:Figure 8. Pressure differenceo2012. Fascicule 3 [July–September] 47