Dynamic working process of the hottest air-cooled

Dynamic simulation and experimental verification of working process of air-cooled heat pump cold and hot water unit under frosting condition

symbol - mass flow, kg/s

Q - heat exchange, kW

s - Laplace transform factor

ρ—— Density, kg/m3

λ—— Gas transmission coefficient, thermal conductivity, w/(m.k)

p - pressure, pa

f - area, m2

h - specific enthalpy, kj/kg

vh - Theoretical exhaust volume, m3/s

t - temperature, k

m - mass, kg

δ—— Gap, thickness, m subscript a - air; In - inlet

out - exit; O - initial

g - vapor; L - liquid 1 if the machine is driven by sprocket, it may slip or the chain may be broken preface in China, the air-cooled heat pump chiller and water heater has been used as the cold and heat source of central air conditioning for more than 10 years. From the perspective of the development process, there is a situation that the theoretical and Experimental Research on the air-cooled heat pump chiller lags behind the application, and there are also some problems in the use of the air-cooled heat pump chiller and water heater, such as the burning of the compressor; The reliability of unit operation stands out among many degradable materials and needs to be further improved; The performance of the unit under frosting condition is not ideal; Poor heating effect; The indicators on the product samples cannot truly reflect the actual operating characteristics of the unit, etc. For this reason, the performance of air-cooled heat pump cold and hot water units is specifically required in the guide for key development products in the mechanical industry issued by the former Ministry of machinery industry: (1) the refrigeration performance coefficient of the unit under nominal working conditions based on Shandong Southeast University is:> 2500w/kw; (2) Stable operation at -10 ℃ ambient temperature; (3) Noise ≤ 74dB; (4) Full computer control, intelligent defrosting. In order to develop products that meet the above requirements, we must strengthen the theoretical and experimental research of air-cooled heat pump cold and hot water units, especially the research on the working characteristics of the units under low temperature and frosting conditions. At present, the difficulty lies in the lack of artificial environmental experimental conditions suitable for the working condition experiment of medium and large-scale air-cooled heat pump cold and hot water units, and the inability to correctly predict the performance of the designed and manufactured products, which seriously restricts the improvement of product quality. Using computer simulation method to study the dynamic and steady-state characteristics of refrigeration system can reduce the dependence on experiments and effectively predict the performance of products [1 ~ 4]. However, at present, no one has used dynamic distributed parameter model to dynamically simulate the working process of medium and large-scale air-cooled heat pump cold and hot water units

therefore, the author establishes the simulation model of the working process of the air-cooled heat pump cold and hot water unit by using the method of combining dynamic centralized parameters with distributed parameters for different components of the air-cooled heat pump cold and hot water unit. The model takes into account the characteristics of the heat exchange in the shell of the fully enclosed compressor and the thermal expansion valve filled with different working fluids. For the wind side heat exchanger, in view of its large structural size, the change of air flow parameters on different levels in the front, and the influence of the development of frost layer on the characteristics of the fan, and the change of the wind speed in the front, using the field experiment of atmospheric environment, the performance change of the air-cooled heat pump cold and hot water unit under the frosting condition [5] is obtained as the calculation basis and compared with the simulation results. 2 Establishment of simulation model of air-cooled heat pump cold and hot water unit 2.1 compressor model

the simulated air-cooled heat pump cold and hot water unit adopts Copland qr15 fully enclosed piston compressor, adopts centralized parameter method to model the compressor, and considers the heat exchange in the shell. The mass flow rate of the compressor is: (1) the heat exchange in the shell of the compressor includes: electrode heat release Q1, compressor friction heat release Q2, refrigerant heat release Q3 through the cylinder wall, high-temperature exhaust heat release Q4 in the shell, shell surface heat release Q5, refrigerant intake heat absorption Q6, and meets the following heat balance equations: (2) 2.2 thermal expansion valve model

the thermal expansion valve filled with tcle10hw heterogeneous working medium of alco company is used by the simulation unit, The problem is that we don't know the working medium filled in the temperature sensing bag. By using the static superheat curve provided by alco and the elastic experiment of the spring, we use the least square method to fit the relationship between the working medium pressure and temperature in the temperature sensing bag: p=4.7583+1.2080 (t+1.0429) +152.9477

× (t+1.0429) 2+2.3116 (t+1.0429) 3

-0.2258 (t+1.0429) 4 (3) using the first-order inertia delay method to express the relationship between the superheat at the outlet of the refrigerant evaporator and the superheat of the working medium in the temperature sensing package [6]: (4) the mass flow through the thermal expansion valve is: (5) 2.3 water side heat exchanger (condenser) and liquid reservoir model

the simulated air-cooled heat pump cold and hot water unit adopts the vertical coil type water side heat exchanger, which is used as a condenser during heating, Because the refrigerant mass flow between the water side heat exchanger and the liquid reservoir should be obtained by solving the two component model simultaneously. Therefore, the two component models are considered together, and the centralized parameter model is adopted for the type of vertical coil heat exchanger

2.3.1 water side heat exchanger mass conservation equation:

liquid phase: