Numerical investigation of laminar flow heat transfer through helically coiled tubes using Al2O3 nanofluid

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Nanofluids have been reported to enhance heat transfer performance in heat exchangers. Additionally, the use of helical coils has shown to be another passive heat transfer enhancement technique. This work presents a CFD modeling study to investigate the laminar heat transfer through helical tubes with nanofluids. The developed CFD models were validated against published experimental results and empirical correlations in the literature. The effects of particles concentration and Reynolds number on heat transfer coefficient were numerically investigated. Results have shown that Al2O3 dispersed in water increases the heat transfer coefficient in helical coils by up to 4.5 times that of pure water in straight tubes at same Reynolds number. For concentrations larger than 2%, Al2O3 is more suitable for thermal systems of small thermal loads where the pumping power is not critical

CFD analysis of single phase flows inside vertically and horizontally oriented helically coiled tubes

It has been well established from previous experimental and numerical work that heat and mass transfer in a helical pipe is higher than that in a corresponding straight pipe.The detailed description of fluid flow and heat transfer inside helical coil is not available from the present literature. This paper clearly shows the variation of average Nusselt number and friction factor with the geometric variables of a vertically and horizontally oriented helical coil for constant wall temperature boundary condition .A comparison of heat transfer and head loss in helical pipe is also discussed for vertical and horizontal orientation. The effect of inlet velocity on heat transfer coefficient and average nusselt number is also described. CFD simulations are carried out for vertically and horizontally oriented helical coils by varying different geometric parameters such as (i) pitch circle diameter, (ii) helical tube pitch and (iii) pipe diameter and their influence on heat transfer and fluid flow has been analysied. After investigate the influence of these parameters, the correlations for average nusselt number and friction factor are developed

Numerical study of an exhaust heat recovery system using corrugated tubes and twisted tape inserts

Thesis (M.S.) University of Alaska Fairbanks, 2014Diesel engine generators are the major power source for small communities in cold regions. Diesel generators waste about 1/3 of their fuel energy in the form of heat through exhaust gas. The primary goal of this work is to capture part of the heat from the exhaust and improve the efficiency of the system. A gas to liquid heat transfer performance of a concentric tube heat exchanger with corrugated tubes and twisted tape inserts is investigated by considering its effects on engine performance and economics. This type of heat exchanger is expected to be inexpensive to install and effective in heat transfer, with minimal effect on exhaust emissions of diesel engines. Most previous research has investigated liquid to liquid heat transfer in corrugated tubes at low Reynolds, not gas to liquid heat transfer. The SolidWorks Flow Simulation computer program was used to perform these studies. The program is first validated by comparing simulation results with renowned correlations and field measurements. Simulations are then conducted for a concentric tube heat exchanger with corrugated tubes and twisted tapes of different configurations to determine the optimal design. The maximum enhancement in the rate of heat transfer was found in an annularly corrugated tube heat exchanger with twisted tape inserts. This exchanger transfers about 235.3% and 67.26 % more heat compared to plain tube and annularly corrugated tube heat exchangers without twisted tapes, respectively. Based on optimal results, for a 120 kWe diesel generator, the application of an annularly corrugated tube heat exchanger with twisted tape inserts can save 2,250 gallons of fuel annually (a cost of approximately $11,330) expected payback of initial cost in one month. In addition, saving heating fuel also reduces CO₂ emissions by 23 metric tons per year

CFD study of an air–water flow inside helically coiled pipes

CFD is used to study an air–water mixture flowing inside helically coiled pipes, being at the moment considered for the Steam Generators (SGs) of different nuclear reactor projects of Generation III+ and Generation IV. The two-phase mixture is described through the Eulerian–Eulerian model and the adiabatic flow is simulated through the ANSYS FLUENT code. A twofold objective is pursued. On the one hand, obtaining an accurate estimation of physical quantities such as the frictional pressure drop and the void fraction. In this regard, CFD simulations can provide accurate predictions without being limited to a particular range of system parameters, which often constricts the application of empirical correlations. On the other hand, a better understanding of the role of the centrifugal force field and its effect on the two-phase flow field and the phase distributions is pursued. The effect of the centrifugal force field introduced by the geometry is characterized. Water is pushed by the centrifugal force towards the outer pipe wall, whereas air accumulates in the inner region of the pipe. The maximum of the mixture velocity is therefore shifted towards the inner pipe wall, as the air flows much faster than the water, having a considerably lower density. The flow field, as for the single-phase flow, is characterized by flow recirculation and vortices. Quantitatively, the simulation results are validated against the experimental data of Akagawa et al. (1971) for the void fraction and the frictional pressure drop. The relatively simple model of momentum interfacial transfer allows obtaining a very good agreement for the average void fraction and a satisfactory estimation of the frictional pressure drop and, at the same time, limits the computational cost of the simulations. Effects of changes in the diameter of the dispersed phase are described, as its value strongly affects the degree of interaction between the phases. In addition, a more precise treatment of the near wall region other than wall function results in a better definition of the liquid film at the wall, although an overestimation of the frictional pressure drop is obtained

Laminar and turbulence forced heat transfer convection correlations inside tubes. A review

This work proposes an extensive review of laminar and turbulent forced convective heat transfer correlations inside tubes by analyzing both experimental and computational research. Convective heat transfer is influenced by fluid turbulence and boundary layers, with geometry significantly impacting flow conditions. Nusselt number correlations quantifying the heat transfer coefficient are vital for various applications. Previous reviews are summarized regarding nanofluid heat transfer modeling approaches in circular tubes. Additionally, comparisons of tube shapes like circular, elliptical and flat geometries are needed. Recent database searches indicate more experimental than numerical studies have been conducted, with greater focus on turbulent flows. This review systematically evaluates laminar and turbulent forced heat transfer correlations reported in the open literature, discussing both older established work as well as newer findings to provide a holistic perspective. The goal is to better understand convective transportation dependencies for optimized thermal design and performance improvement.Comment: 42 Pages, 14 figure

Pulsatile Flow in A Helically Coiled Tube-in-Tube Heat Exchanger

Heat exchangers have achieved extensive applications in modern days in every field. They have got wide applications in automobile industries, food processing operations, aerospace applications. Among different types of heat exchangers, helical tube heat exchangers have gained much popularity due to their compactness and effectiveness. Helical heat exchngers provide greater surface area for heat exchange for same floor space. In this paper pulsating flow(both parallel and counter flow) is studied in a model of a helical tube-in-tube heat exchanger using ANSYS 15.0. The results are compared with constant flow(both parallel and counter flow) in the same model of the helical tube-in-tube heat exchanger. Then further analysis is done by studying pulsating flow (both parallel and counter flow) in the same model of helical tube-in-tube heat exchanger but by fixing a thin wire on the surface of the inner helical tube of the heat exchanger model. All analysis is done using ANSYS 15.0 for flow time of 60 seconds in each case. The temperature drops for the hotter fluid and temperature rise for the colder fluid is studied for each case and then comparision is done to establish the best configuration of the above. It is observed that the temperature drop for the hot fluid and temperature rise for cold fluid is: Constant velocity flow < Pulsating flow < Pulsating flo

Numerical investigation of turbulent flow heat transfer and pressure drop of AL2O3/water nanofluid in helically coiled tubes

Passive convective heat transfer enhancement can be achieved by improving the thermo-physical properties of the working fluid, changing flow geometry or both. This work presents a numerical study to investigate the combined effect of using helical coils and nanofluids on the heat transfer characteristics and pressure losses in turbulent flow regime. The developed computational fluid dynamics models were validated against published experimental data and empirical correlations. Results have shown that combining the effects of alumina (Al2O3) nanoparticles and tube coiling could enhance the heat transfer coefficient by up to 60% compared with that of pure water in straight tube at the same Reynolds number. Also, results showed that the pressure drop in helical coils using Al2O3 nanofluid for volume fraction of 3% was six times that of water in straight tubes (80% of the pressure drop increase is due to nanoparticles addition), while the effect of Reynolds number on the pressure drop penalty factor was found to be insignificant

Contrastive study of flow and heat transfer characteristics in a helically coiled tube under uniform heating and one-side heating

One-side heated helically coiled tubes, which are generally applied in various industrial applications such as the water cooled wall in power plant boilers though, have not been thoroughly studied. To investigate the flow and heat transfer characteristics in this case, numerical simulation of the flow in a helically coiled tube is performed under uniform and non-uniform (heating on the inner coil side wall) heat flux boundary conditions for both laminar and turbulent flows. Temperature distributions, secondary flow distributions, average Nusselt number variation with respect to Reynolds number and local Nusselt number along the periphery on the wall in the fully developed section are discussed contrastively under the two different heating conditions. It is found that the secondary flow distributions are hardly affected by changing heating method, however, a larger temperature gradient can be found for one-side heating condition. The average Nusselt numbers are close for laminar flow under the two heating methods, but one-side heating shows 7–10% lower average Nusselt numbers than uniform heating for turbulent flow, thus a new correlation of average Nusselt number for turbulent flow and one-side heating is proposed. Furthermore, a special point on the inner wall where the local Nusselt numbers are almost the same when carrying out different heating conditions in laminar and turbulent flows is found, which should be useful for measuring unknown parameters

A review on the two-phase pressure drop characteristics in helically coiled tubes

Due to their compact design, ease of manufacture and enhanced heat transfer and fluid mixing properties, helically coiled tubes are widely used in a variety of industries and applications. In fact, helical tubes are the most popular from the family of coiled tube heat exchangers. This review summarises and critically reviews the studies reported in the pertinent literature on the pressure drop characteristics of two-phase flow in helically coiled tubes. The main findings and correlations for the frictional two-phase pressure drops due to: steam-water flow boiling, R-134a evaporation and condensation, air-water two-phase flow and nanofluid flows are reviewed. Therefore, the purpose of this study is to provide researchers in academia and industry with a practical summary of the relevant correlations and supporting theory for the calculation of the two-phase pressure drop in helically coiled tubes. A significant scope for further research was also identified in the fields of: air-water bubbly flow and nanofluid two phase and three-phase flows in helically coiled tubes