Effectiveness - NTU data and analysis for air conditioning and refrigeration air coils

A simulation program based on a control volume analysis has been used in the evaluation of the (ε, NTU) relationship for coils of complex geometry and flow arrangement. The simulation program has been evaluated through simple geometry and flow arrangement coils. The program results compare very well with correlations for simple cross flow coils, and a number of rows up to four. It has also been determined that closed form correlations developed for coils of an infinite number of tube rows are inadequate for those with number of rows in the range between 5 and 10. In addition, it has been found that closed form (ε, NTU) correlations for cross flow coils with the same tube arrangement and number of rows might lead to inaccuracies higher than 10% in the evaluation of the effectiveness of coils of complex flow arrangement

Simulation of Boiling Heat Transfer at Different Reduced Temperatures with an Improved Pseudopotential Lattice Boltzmann Method

[Abstract] The pseudopotential Lattice Boltzmann Method has attracted much attention in the recent years for the simulation of boiling heat transfer. Many studies have been published recently for the simulation of the bubble cycle (nucleation, growth and departure from a heated surface). This paper puts forward two-dimensional simulations of bubble nucleation, growth and departure using an improved pseudopotential Lattice Boltzmann Model from the literature at different reduced temperatures, Tr=0.76 and Tr=0.86. Two different models using the Bhatnagar–Gross–Krook (BGK) and the Multiple-Relaxation-Time (MRT) collision operators with appropriate forcing schemes are used. The results for pool boiling show that the bubbles exhibit axial symmetry during growth and departure. Numerical results of departure diameter and release period for pool boiling are compared against empirical correlations from the literature by varying the gravitational acceleration. Reasonable agreement is observed. Nucleate boiling trends with heat flux are also captured by the simulations. Numerical results of flow boiling simulations are compared by varying the Reynolds number for both reduced temperatures with the MRT model. It was found that the departure diamenter and release period decreases with the increase of the Reynolds number. These results are a direct effect of the drag force. Proper conclusions are commented at the end of the paper.Brasil. Conselho Nacional de Desenvolvimento Científico e Tecnológico; 304972/2017-7Brasil. Coordenação de aperfeiçoamento de pessoal de nivel superior; 001Fundação de Amparo à Pesquisa do Estado de São Paulo; 2016/09509-1Fundação de Amparo à Pesquisa do Estado de São Paulo; 2018/09041-5All the authors fully acknowledge the support provided by CNPq (National Council for Scientific and Technological Development, process 304972/2017-7), CAPES (Coordination for the Improvement of Higher Education Personnel, Finance Code 001) and FAPESP (São Paulo Foundation for Research Support, 2016/09509-1 and 2018/09041-5)

Flow boiling heat transfer of R134a and low GWP refrigerants in a horizontal micro-scale channel

The present paper presents an investigation of the effects of the refrigerant type on the heat transfer coefficient during flow boiling inside micro-scale channels. Experimental results for R134a, R1234ze(E), R1234yf and R600a for flow boiling in a circular channel with internal diameter of 1.1 mm are presented. The experimental database comprises 3409 data points covering mass velocities ranging from 200 to 800 kg/m²s, heat fluxes from 15 to 145 kW/m², saturation temperatures of 31 and 41°C, and vapor qualities from 0.05 to 0.95. The experimental data were parametrically analysed and the effects of the experimental parameters (heat flux, mass velocity, saturation temperature and working fluid) identified. Subsequently, the experimental data were compared against the most quoted predictive methods from literature, including macro and micro-scale methods. Based on the broad database obtained in the present study, an updated version of the predictive method of Kanizawa et al. [1] was proposed. The updated version provided accurate predictions of the present experimental database, predicting more than 97% and 86% of the results within error bands of ±30 and ±20%, respectively.The authors gratefully acknowledge FAPESP (The State of São Paulo Research Foundation, Brazil) for the financial support under contract numbers 2010/17605-4 and 2011/50176-2 and CNPq (The National Council for Scientific and Technological Development, Brazil) for the financial support under Contract Numbers nº476763/2013-4 and 303852/2013-5. The technical support given to this investigation by Mr. José Roberto Bogni is also appreciated and deeply recognized. The authors are also grateful to Honeywell for supplying the low GWP refrigerants R1234ze(E) and R1234yf

Theoretical and experimental analysis of pool boiling of halocarbon refrigerants

O estudo consistiu em uma análise teórica experimental, com o desenvolvimento de uma correlação, dos parâmetros que afetam a ebulição nucleada de refrigerantes halogenados em superfícies cilíndricas lisas. Uma ampla análise da literatura permitiu levantar os distintos tópicos envolvendo este tema destacando entre eles o presente estudo. A análise da literatura envolveu, ainda, um estudo detalhado dos fundamentos da ebulição e das correlações para a previsão do coeficiente de transferência de calor. Foi projetado e construído um aparato experimental no qual foram realizados ensaios envolvendo superfícies de aço inoxidável, cobre e latão, os refrigerantes R-11, R-123, R-12, R-22 e R-134a, fluxos específicos de calor entre 0,8 e 120 kW/m&sup2, pressões reduzidas de 0,008 a 0,26 e rugosidade aritmética média variando entre 0,02 e 3,3 &#956m. A análise destes resultados possibilitou a verificação de comportamentos físicos inéditos segundo a literatura consultada. Destaca-se a análise de efeitos da condução longitudinal e da resistência térmica de contato entre a superfície de transferência de calor e os termopares. Esta análise foi incorporada ao procedimento de determinação da temperatura superficial, e permitiu estabelecer a distribuição de temperaturas ao longo do perímetro da superfície. Finalmente, com base no banco de dados levantado, foi desenvolvida uma correlação para o coeficiente de transferência de calor em ebulição nucleada plenamente desenvolvida de refrigerantes halogenados.The present research has been focused in a theoretical and experimental analysis of the parameters that affect the pool boiling of halocarbon refrigerants in smooth cylindrical surfaces with the development of a correlation. A wide survey of the literature allowed to list the distinct topics related to this subject and to highlight among them the theme of this study. The literature analysis still involved a detailed study of the pool boiling fundamentals and of the correlations for the pool boiling heat transfer coefficient. An experimental set up has been developed and constructed in which were raise data involving surfaces of stainless steel, copper and brass, the refrigerants R-11, R-123, R-12, R-22 and R-134a, specific heat flux range of 0.8 to 120 kW/m&sup2, reduced pressures from 0.008 to 0.26 and arithmetic mean roughness varying between 0.02 and 3.3 &#956m. The analysis of these results revealed, according to the consulted literature, unpublished physical behaviors. In this work the effects of the longitudinal conduction and of the thermal contact resistance between the heat transfer surface and the thermocouple were analyzed carefully. This analysis was incorporated to the superficial temperature determination procedure and allowed to establish the temperature distribution along the surface perimeter. Finally, using the data base raised in this research, a simple and accurate correlation for the fully developed pool boiling heat transfer coefficient for halocarbon refrigerants applications has been developed

Theoretical and experimental analysis of pool boiling of halocarbon refrigerants

O estudo consistiu em uma análise teórica experimental, com o desenvolvimento de uma correlação, dos parâmetros que afetam a ebulição nucleada de refrigerantes halogenados em superfícies cilíndricas lisas. Uma ampla análise da literatura permitiu levantar os distintos tópicos envolvendo este tema destacando entre eles o presente estudo. A análise da literatura envolveu, ainda, um estudo detalhado dos fundamentos da ebulição e das correlações para a previsão do coeficiente de transferência de calor. Foi projetado e construído um aparato experimental no qual foram realizados ensaios envolvendo superfícies de aço inoxidável, cobre e latão, os refrigerantes R-11, R-123, R-12, R-22 e R-134a, fluxos específicos de calor entre 0,8 e 120 kW/m&sup2, pressões reduzidas de 0,008 a 0,26 e rugosidade aritmética média variando entre 0,02 e 3,3 &#956m. A análise destes resultados possibilitou a verificação de comportamentos físicos inéditos segundo a literatura consultada. Destaca-se a análise de efeitos da condução longitudinal e da resistência térmica de contato entre a superfície de transferência de calor e os termopares. Esta análise foi incorporada ao procedimento de determinação da temperatura superficial, e permitiu estabelecer a distribuição de temperaturas ao longo do perímetro da superfície. Finalmente, com base no banco de dados levantado, foi desenvolvida uma correlação para o coeficiente de transferência de calor em ebulição nucleada plenamente desenvolvida de refrigerantes halogenados.The present research has been focused in a theoretical and experimental analysis of the parameters that affect the pool boiling of halocarbon refrigerants in smooth cylindrical surfaces with the development of a correlation. A wide survey of the literature allowed to list the distinct topics related to this subject and to highlight among them the theme of this study. The literature analysis still involved a detailed study of the pool boiling fundamentals and of the correlations for the pool boiling heat transfer coefficient. An experimental set up has been developed and constructed in which were raise data involving surfaces of stainless steel, copper and brass, the refrigerants R-11, R-123, R-12, R-22 and R-134a, specific heat flux range of 0.8 to 120 kW/m&sup2, reduced pressures from 0.008 to 0.26 and arithmetic mean roughness varying between 0.02 and 3.3 &#956m. The analysis of these results revealed, according to the consulted literature, unpublished physical behaviors. In this work the effects of the longitudinal conduction and of the thermal contact resistance between the heat transfer surface and the thermocouple were analyzed carefully. This analysis was incorporated to the superficial temperature determination procedure and allowed to establish the temperature distribution along the surface perimeter. Finally, using the data base raised in this research, a simple and accurate correlation for the fully developed pool boiling heat transfer coefficient for halocarbon refrigerants applications has been developed

Two-phase pressure drop during upward cross flow in triangular tube bundle

This paper presents experimental results for pressure drop during air-water upward two-phase flow across horizontal triangular tube bundle. It is estimated that more than half of shell-and-tube heat exchangers in industry operate under two-phase flow conditions in the shell side. However the number of research and publications focused on external flow is considerably reduced when compared to intube flow. This study addresses experimental results for pressure drop during external flow of air and water mixtures across a triangular tube bundle counting with 19 mm OD tubes and 24 mm transverse pitch, for superficial velocities up to 0.553 and 10 m/s for water and air, respectively. For reduced mass velocities, the gravitational pressure drop parcel is dominant, consequently the adoption of an appropriate methodology for void fraction estimative is essential for accurate estimative of the gravitational pressure drop parcel. The experimental results are compared with predictive methods available in the open literature, and an analysis of this comparison is presented

Two-Phase Frictional Pressure Drop and Flow Boiling Heat Transfer for R245fa in a 2.32-mm Tube

Experimental two-phase frictional pressure drop and flow boiling heat transfer results are presented for a horizontal 2.32-mm\ud ID stainless-steel tube using R245fa as working fluid. The frictional pressure drop data was obtained under adiabatic and\ud diabatic conditions. Experiments were performed for mass velocities ranging from 100 to 700 kg m−2 s−1\ud , heat flux from 0\ud to 55 kW m−2\ud , exit saturation temperatures of 31 and 41◦C, and vapor qualities from 0.10 to 0.99. Pressures drop gradients\ud and heat transfer coefficients ranging from 1 to 70 kPa m−1 and from 1 to 7 kW m−2 K−1 were measured. It was found that\ud the heat transfer coefficient is a strong function of the heat flux, mass velocity, and vapor quality. Five frictional pressure\ud drop predictive methods were compared against the experimental database. The Cioncolini et al. (2009) method was found\ud to work the best. Six flow boiling heat transfer predictive methods were also compared against the present database. Liu\ud and Winterton (1991), Zhang et al. (2004), and Saitoh et al. (2007) were ranked as the best methods. They predicted the\ud experimental flow boiling heat transfer data with an average error around 19%.Proc. 05/60031-0, 06/52089-1, 07/53950-5 - FAPES