Numerical methods in laminar and turbulent flow

As per the Montreal Protocol, CFCs and HCFCs are being phased out. HCFC-22 is used in window air conditioners. This paper presents the experimental performance study of a window air conditioner with propane (HC-290), a natural refrigerant, as a drop-in substitute to HCFC-22. Experimental results showed that HC-290 had 6.6% lower cooling capacity for the lower operating conditions and 9.7% lower for the higher operating conditions with respect to HCFC-22. The coefficient of performance for HC-290 was 7.9% higher for the lower operating conditions and 2.8% higher for the higher operating conditions. The energy consumption of the unit with HC-290 was lower in the range 12.4–13.5% than HCFC-22. The discharge pressures for HC-290 were lower in the range 13.7–18.2% than HCFC-22. For HC-290, the pressure drop was lower than HCFC-22 in both heat exchangers. This paper also presents simulation results for the heat exchangers of an HCFC-22 window air conditioner with HC-290 as a drop-in substitute. The simulation has been carried out using EVAP-COND, a heat exchanger model developed by NIST [National Institute of Standards and Technology. EVAP-COND: simulation models for finned-tube heat exchangers, Maryland, USA (2003). http://www2.bfrl.nist.gov/software/evap-cond/ [18] ]. The simulated evaporator capacities are within ±4% of the experimentally measured cooling capacities for both refrigerants. Simulation results for HC-290 and HCFC-22 are compared. The exit temperatures of HC-290 are lower by 0.3–1.2 °C in the condenser and are higher by 2.1–2.4 °C in the evaporator than HCFC-22. Evaporating pressures of HC-290 are lower by 2.1–3.3% as compared to HCFC-22. The pressure drops of HC-290 are lower in both the evaporator and the condenser as compared to HCFC-22. The outlet temperatures of air for HCFC-22 and HC-290 in both heat exchangers are nearly the same.

Performance assessment of HC-290 as a drop-in substitute to HCFC-22 in a window air conditioner.

Performance assessment of HCFC-22 window air conditioner retrofitted with R-407C

The use of CFD for predicting and optimizing the performance of air conditioning equipment

A comparison of the predicted and experimental heat transfer performance of a finned tube evaporator

Computational fluid dynamics (CFD) and heat transfer has matured to the stage where state-of-the-art commercial codes can address real engineering problems. However, the advances in finite volume numerical capability which permit this, such as full three-dimensionality, generalised coordinate systems and multiblocking, require convincing validation procedures to demonstrate quantitative engineering potential. In this paper a two-dimensional incompressible turbulent flow in an industrial packaged air-conditioning system is investigated numerically, with specific reference to the air flow through the tube banks of the evaporator coil. The system modelled comprises an outer casing containing the coil which has three rows of 20-mm diameter tubes in staggered formation, each row having tubes, and a condensate tray with complex geometry. The analysis is based on the Reynolds-averaged Navier-Stokes equations in conjunction with the standard k-ϵ turbulence model. The finite-volume method (FVM) is used to solve the governing partial differential equations on a non-staggered grid. In order to treat the detailed fin tube geometry, a novel method called ‘masking’ has been developed to construct body-fitted curvilinear grids quickly and efficiently. In the computation, several differencing schemes of various orders are outlined and their accuracy are examined. In a parallel experimental investigation conducted on the actual air-conditioning unit, both the mean velocity profiles and turbulence properties of the flow were obtained from triple hot-wire anemometry measurements. These were then used to compare with the computational results and validate the mathematical models developed. It is found that the predictions were generally in good agreement with the measurements, especially for the higher-order differencing scheme. An immediate practical effect of the study was that it revealed the flow disturbance resulting from the condensate tray. This has been suspected but its magnitude not anticipated. The consequent amendment of the design of this component in the commercial unit led to an improved performance of the evaporator.

A numerical and experimental study of turbulent flow through the evaporator coil in an air-conditioning unit

The complex three-dimensional turbulent flow through an industrial packaged air-conditioning unit was studied numerically in order to investigate the occurrence of non-uniformities in the flow, which frequently arise in such systems. One main cause of this phenomenon can be attributed to the location of the wide variety of components, such as a heat exchanger, humidifier and fan, that are installed into a casing where space is limited. In addition, regions of recirculation may occur in the wake of these components. These two effects may be responsible for a reduction in the performance of the system. This study is based on the solution of the Reynolds-averaged Navier-Stokes equations for the mean flow and a two-equation k-e model for the turbulence. To minimise excessive computational demands, the heat exchanger coil is modelled as a distributed flow resistance element. As regards the flow through the rotating fan impellers, Coriolis and centripetal forces are included in the momentum equations as source ...

CFD Prediction of Turbulent Recirculating Flow in an Industrial Packaged Air-Conditioning Unit

SUMMARY Details are given of a study to obtain experimental data in an idealized environment for the purpose of evaluating the corresponding computational predictions and which supplement parallel measurements made in actual packaged air-conditioning units. The system consisted of a purpose-built low-speed wind tunnel with a working section constructed to reproduce particular features of the real units. In the experiment, both the mean velocity profiles and turbulence properties of the flow are obtained from triple-hot-wire anemometry measurements. A numerical model, based on finite volume methodology, was used to obtain the solution of the Reynolds-averaged Navier‐Stokes equations for incompressible isothermal flow. The Reynolds stress terms in the equations are calculated using the standard k ‐e model and second-moment closure (Reynolds stress) models. The accuracy of the two models was evaluated against the experimental measurements made 10 mm downstream of a baffle. The results show that the standard k‐e model gave the better agreement except in regions of strong recirculation. © 1998 John Wiley & Sons, Ltd.

Z. G. Xu

Papers

A numerical and experimental study of turbulent flow through the evaporator coil in an air-conditioning unit

CFD Prediction of Turbulent Recirculating Flow in an Industrial Packaged Air-Conditioning Unit

Validation of turbulence models in a simulated air‐conditioning unit