Pressure loss and performance assessment of horizontal spiral coil inserted pipes during forced convective evaporation of R-600a

The impacts of spiral coil inserts on the pressure loss of environment-friendly refrigerant R-600a are experimentally evaluated during forced convective evaporation within horizontal copper pipes. Then, by considering the current pressure drop and previous heat transfer results and calculating the performance factor, the overall effectiveness of the inserts is determined. Experiments are carried out for smooth and rough tubes. Five spiral coils with various coil pitches and wire diameters are utilized. Also, vapor qualities between 0.08-0.7 and mass fluxes between 109.2-505 kgm-2s-1 are considered for tests. Generally, it is observed that inserts augment the pressure loss in the range of 90-958% over the smooth pipe. However, as the wire diameter decreases and the coil pitch increases, less pressure losses are imposed to the system. Depending on the operational conditions and inserts type, the performance factor was obtained between 0.13-1.40. The coiled wire “CW5” with the wire diameter of 1 mm and coil pitch of 30 mm performed superior compared to the other inserts by maximum performance factor of 1.4. Finally, by using the current data a new correlation is proposed in order to predict the evaporative pressure loss of R-600a within spiral coil inserted pipes

Evaluation of R448A and R450A as Low-GWP alternatives for R404A and R134a using a micro-fin tubes evaporator model

[EN] When retrofitting new refrigerants in an existing vapour compression system, their adaptation to the heat exchangers is a major concern. R450A and R448A are commercial non-flammable mixtures with low GWP developed to replace the HFCs R134a and R404A, fluids with high GWP values. In this work the evaporator performance is evaluated through a shell-and-microfin tube evaporator model using R450A, R448A, R134a and R404A. The accuracy of the model is first studied considering different recently developed micro-fin tube correlations for flow boiling phenomena. The model is validated using experimental data from tests carried out in a fully monitored vapour compression plant at different refrigeration operating conditions. The main predicted operational parameters such as evaporating pressure, UArp, and cooling capacity, when compared with experimental data, fit within 10% using the Akhavan-Behabadi et al. correlation for flow boiling. Results show that R450A and R404A are the refrigerants in which the model fits better, even though R448A and R134a predictions are also accurate. (C) 2015 Elsevier Ltd. All rights reserved.The authors thankfully acknowledge "Ministerio de Educacion, Cultura y Deporte" (Grant number FPU12/02841) for supporting this work through "Becas y Contratos de Formacion de Profesorado Universitario del Programa Nacional de Formacion de Recursos Humanos de Investigacion del ejercicio 2012". Finally the linguistic support of Irene I. Elias-Miranda is appreciated.Mendoza Miranda, JM.; Mota-Babiloni, A.; Navarro Esbri, J. (2016). Evaluation of R448A and R450A as Low-GWP alternatives for R404A and R134a using a micro-fin tubes evaporator model. Applied Thermal Engineering. 98:330-339. https://doi.org/10.1016/j.applthermaleng.2015.12.064S3303399

Entropy analysis on convective film flow of power-law fluid with nanoparticles along an inclined plate

Entropy generation in a two-dimensional steady laminar thin film convection flow of a non-Newtonian nanofluid (Ostwald-de-Waele-type power-law fluid with embedded nanoparticles) along an inclined plate is examined theoretically. A revised Buongiorno model is adopted for nanoscale effects, which includes the effects of the Brownian motion and thermophoresis. The nanofluid particle fraction on the boundary is passively rather than actively controlled. A convective boundary condition is employed. The local nonsimilarity method is used to solve the dimensionless nonlinear system of governing equations. Validation with earlier published results is included. A decrease in entropy generation is induced due to fluid friction associated with an increasing value of the rheological power-law index. The Brownian motion of nanoparticles enhances thermal convection via the enhanced transport of heat in microconvection surrounding individual nanoparticles. A higher convective parameter implies more intense convective heating of the plate, which increases the temperature gradient. An increase in the thermophoresis parameter decreases the nanoparticle volume fraction near the wall and increases it further from the wall. Entropy generation is also reduced with enhancement of the thermophoresis effect throughout the boundary layer

Mixed convection heat transfer and pressure drop characteristics of the copper oxide-heat transfer oil (CuO-HTO) nanofluid in vertical tube

In this paper, the mixed natural-forced convection is experimentally investigated for the heat transfer oil-copper oxide (HTO-CuO) nanofluid flow upward in a vertical tube. The flow regime is laminar and the temperature of the tube surface is constant. The effect of the nanoparticles concentration on the heat transfer rate and the pressure drop is studied as Richardson number varies between 0.1 and 0.7. It is observed that the mixed convection heat transfer rate increases with both the nanoparticles concentration and Richardson number. New correlations are proposed to predict the Nusselt number of the nanofluid flow with the reasonable accuracy. As the heat transfer enhancement methods usually accompany with increment in the pressure drop, the figure of merit is evaluated experimentally. As such the maximum figure of merit of 1.31 is achieved using the 1.5% concentration of the nanoparticles in Richardson number of 0.7. This study provides a platform to design next generation of low flow rate nanofluid-based heat exchangers and may improve the accuracy of predicting the mixed convection characteristics of nanofluid flows