Analysis of the influence of the heat transfer model on the effectiveness of the heat exchanger on the example of cross flow device

Influence of flow shedding frequency on convection heat transfer from bank of circular tubes in heat exchangers under cross flow

Experiments were conducted on cold tube banks subjected to a crossflow of water. The tubes were internally cooled below the freezing temperature and became enveloped in ice. The resulting ice shapes, which formed on the outside surfaces of the tubes, were allowed to stabilize and their impact on the total steady-state rate of energy exchange between the tubes and the flowing water was investigated. Both in-line and staggered tube bank geometries were considered with tests conducted in the low to moderate Reynolds Number range (Red = 100 - 2000) and for cooling temperature ratio variations of 0.5 < Θ < 8. The ice formations were directly observed and photographed, and the total heat transfer rate for the tube bank was inferred from a simple energy balance on the system. The ice shapes which formed around the tubes were characterized as falling into one of three distinct categories: ice formations with no linkage occurring between any adjacent tubes; ice formations with partial linkage of some adjacent tubes; and, for the staggered tube bank, a complete linkage of a majority of the tubes. The experiments showed that the ice formations dramatically affected the convective heat transfer rate of the tube banks (when compared to non-icing tube banks at the same Red) and that the change in heat transfer rate is dependent on the tube bank geometry. In the no-link category, the ice formations were found to either increase or decrease the tube bank heat transfer rate depending on the amount of ice build-up accumulated; the staggered configuration showing a greater overall rise with Θ than the in-line geometry. Ice linkage between adjacent tubes was found to be detrimental to the heat transfer rate of the staggered bank, however, the same phenomena on the in-line tube bank did not seriously impede its heat transfer rate. Correlations expressing the heat transfer behavior of both in-line and staggered tube banks with ice formations at steady state have been developed.

Tube bank heat exchangers are designed to efficiently transfer heat between two fluids. Shapes and arrangements of tubes in heat exchangers have significant effects in heat transfer and pressure drop of fluid. In this study, the three-dimensional (3D) numerical investigation is performed to determine heat transfer coefficients, friction factor, and performance evaluation criteria (PEC) of cam-shaped tube banks in aerodynamic and inverse aerodynamic directions in the cross flow air and compared with those of elliptical tube banks in heat exchanger. The arrangements of tubes are aligned and staggered with longitudinal pitch of 44.88 mm and transverse pitch of 28.05 mm. Reynolds number in the range of 11,500–18,500 was used, and the tube surface temperature was fixed and considered 352 K. Results indicate the superior heat transfer of elliptical tube bank over the cam-shaped tube banks in inverse aerodynamic and aerodynamic directions in both arrangements. Moreover, the PEC of the cam-shaped tube banks with inverse aerodynamic and aerodynamic directions and elliptical tube bank in aligned arrangement are approximately 1.4, 1.1, and 1.6, respectively. The obtained results for staggered arrangements are also 1.5, 1.3, and 1.8, respectively.

Convective heat transfer for cold tube bundles with ice formations in a stream of water at steady state

Condenser is a type of heat exchanger that serves as condensation for turbine output steam. In general, a steam power plant uses a surface condenser. This type of condenser is a type of shell and tube. Condensers have a phenomenon of convection heat transfer. The change in distance between condenser tubes is one way to increase the rate of heat transfer. To produce maximum heat transfer, it can be determined by the variety of distances between tubes. The research was conducted by analyzing the flow characteristic and heat transfer around condenser tube banks with gap ratio variation (s/d), which compares transverse distance (ST ) and tube diameter (d) with variations of 3.2, 2.7, 2, and 1.7. 2-dimensional simulation with Computational Fluid Dynamics (CFD) is a numerical method and algorithm for solving and analyzing problems that occur in fluid flows. Based on the results obtained, the smaller the Gap Ratio, the lower the outlet temperature produced and the maximum heat transfer, but the pressure drop value obtained the bigger. Also, the smaller Gap ratio, the bigger v max produced. The lowest outlet temperature value at the variation of 1.7 is 310.66 K, and the coefficient of heat transfer is 103.34 W/m2K. Pressure drop produced at variation 1.7 is 515.99 Pa. Variation 2 is the best gap ratio variation, seen from the heat transfer result and the pressure drop value is not too large. The coefficient heat transfer and pressure drop in variation 2 are 76.95 W/m2K and 119.17 Pa.

The present work aims to introduce 2-D numerical investigations of turbulent flow over staggered tube bank by using a commercial CFD package (ANSYS-Fluent). The realizable k-e turbulent model is applied. The effect of the longitudinal pitch ratio (SL/D), transverse pitch ratio (ST/D), Reynolds number, and tube shape on the heat transfer and flow characteristics is studied. Due to symmetry, half of the tube bank rows are considered. The present simulation results are compared with the available experimental data and previous analytical results, which show acceptable agreement. The results of circular tube bank show that there is an enhancement on the heat transfer ranging from 1.02 to 8.8% when the longitudinal pitch ratio changed from 1.25 to 3 at ST/D = 1.25 and 5000 ≤ Re ≤ 50,000. Additionally, the friction factor decreases for ST/D ≥ 2 as the longitudinal pitch ratio increases but it increases for ST/D ≤ 1.5. For flat tube bank, the results show that as the length ratio decreases the heat transfer increases. Moreover, the flat tube diameter should be small to obtain the highest heat transfer. Furthermore, two different correlations are obtained for Nusselt number for the two tube banks.

An application of tubes with punched spiral-tape finning is promising for heat exchange intensification in convective heating surfaces of boilers and boilers-utilizers. Results of experimental research into thermo-aerodynamic characteristics of specified heating surfaces are presented. As a result of research, heat exchange intensification by 17…32 % due to fin punching was established. Heat transfer increases at an increase in the Reynolds number and a decrease in the degree of finning. Dependence of heat transfer on the accepted parameter for characterizing the bank geometry – relative to longitudinal pitch of tubes – is extreme with peaks in domain of variability relative to longitudinal pitch of 2.7…3.5. Efficiency of punched finning was determined. Results of the study of aerodynamic resistance showed its increase by 18…40 % due to the punching of fins. Resistivity increases at an increase in reduced length of the extended surface, and decreases at an increase in ratio of transverse pitch of tubes of the bank to longitudinal pitch and in Reynolds number. Results of experiment were generalized and formulas for engineering calculations of heat exchange and aerodynamic resistance of in-line tube banks with punched spiral finning were proposed. The formulae hold in domains of variability of defining parameters: finning coefficient y=6.01…9.012, relative longitudinal pitch of tubes in banks s 2 =2…6, Reynolds numbers Re d =6·103…4·104 and Re e =5·10 3 …4·10 4 , ratios of tube pitches S 1 /S 2 =0.4…2.5 and reduced length of extended surface – H/F=4.58…30.45. We established the intervals variability in ratio of pitches of tubes, in which thermo-aerodynamic efficiency of in-line and staggered tube banks is maximal, respectively: 1.0...1.5 and 2.0...3.0. Within these intervals, values of the Kirpichov criterion are, respectively, E=125...150 for in-line and 75...80 for staggered banks. Formulae establish relationship between Nusselt and Euler criterion with geometric characteristics of banks and Reynolds numbers. We determined thermo-aerodynamic efficiency of in-line and staggered banks of tubes with punched spiral finning by results of experimental studies. In-line banks have higher efficiency. As a result of calculation research into thermo-aerodynamic efficiency of four types of heating surfaces of a powerful boiler-utilizer, in-line tube banks with punched spiral finning turned out to be more efficient by this parameter. The Kirpichov criterion for these tube banks, located in one shell, is 319, for staggered tube banks with punched finning – 228.8, for staggered tube banks with continuous finning – 223.8, and for staggered bare-tube banks – 143.0.

Experimental and numerical studies on the air-side flow and heat transfer characteristics of a novel heat exchanger

Adiabatic compressed air energy storage technology

In the present study, combined convective and radiative heat transfer from in-line tube banks placed in an absorbing, emitting and isotropically scattering participating medium was numerically investigated. The governing flow and radiative transfer equations were solved numerically by utilizing the finite volume methodology and the P1 approximation. The flow is laminar, and the tube walls are black and isothermal. The parameters affecting the heat transfer in such media are the tube pitch, scattering albedo, Reynolds number and conduction-to-radiation parameter. Numerical simulations were carried out for Reynolds numbers between 100 and 300, and the transversal and longitudinal pitches ranging from 1.5D to 3-D, respectively. Also, the medium related parameters such as scattering albedo and conduction-to-radiation parameter were ranged between 0 to 1 and 0.0367 to 0.5080, respectively. The effects of the change in scattering albedo, conduction-to-radiation parameter as well as flow characteristic and tube pitch on temperature distribution and heat transfer were examined. The convective and radiative heat transfer from tubes is analyzed via the surface averaged mean Nusselt number over the whole tube wall. The lowest and the highest total-radiative Nusselt number values were achieved as 3.66-0.091 in the purely scattering medium and as 32.95-25.14 in the non-scattering medium, respectively. Also, a novel correlation (with a coefficient of determination of 0.99804) for tube banks subjected to convection and radiation in a participating media was developed.

Experimental research on enhanced heat transfer of double-pipe exchanger with audible acoustic field

The paper presents a gas-water heat exchanger based on the technology of a microheat pipe. The heat exchanger parameters, such as inlet and outlet temperature of flue gas, wind speed and inlet and outlet temperature of the water, were measured in practice. The analysis data showed that the temperature difference between the flue gas and the flue gas before and after heat exchange increased with the inlet temperature. At the same time, the water temperature difference remained unchanged. This paper analyzed the influence of different inlet temperatures and wind speed on heat transfer using the control variable method. It was found that the inlet temperature of flue gas was 190°C and the heat transfer efficiency was stable above 0.7 when the heat transfer was maximum, which was higher than the national standard. The irreversibility of the heat transfer process was analyzed by introducing the dimensionless equivalent thermal resistance R* and the thermal conductivity N*, which were derived from the dissipation number of the entransy. It was found that the heat exchanger operated stably when the flue gas temperature and the wind speed were constant. However, when the flue gas temperature or wind speed was increased, it was appropriate to use a lower wind speed to maintain the heat transfer performance before the temperature reached the critical temperature. After getting the critical temperature, the heat transfer performance was compared with the effective degree-NTU method and the effective degree-thermal conductivity method.

Convection heat transfer in a shell-and-tube heat exchanger using sheet fins for effective utilization of energy

Numerical optimization of heat and fluid flow over staggered tube banks are investigated for both unfinned and axially finned tube banks. Ansys Fluent software is utilized for numerical computations. To find maximum total heat transfer and minimum volume of tube bank for the given mass flow rate, allowable pressure drop and tube bank effectiveness; A Multi-objective Genetic Algorithm II is used. Optimum designs are given on tabular forms and variations of total heat transfer with total volume are presented. Moreover, effects of transversal and longitudinal pitches on objective functions are graphically demonstrated. In order to confirm reliability of the numerical studies, unfinned staggered tube bank configuration is compared with experimental studies in literature. As a result of optimization studies, axial fin usage results in 42.7% decrement in volume of staggered tube bank with respect to unfinned case, while heat transfer values are equal with maximum obtainable heat transfer rate for the given optimization problem.

Tube banks are common design elements in heat exchangers. The most common tube bank patterns are In-line tube banks and Staggered tube banks. These are characterized by their longitudinal and transverse pitch ratios (pitches to diameters). The tube bank with a pitch ratio of less than 1.25 is considered a compact tube bank and a pitch ratio greater than 2 is considered as widely spaced. In this paper, the attention is focused on the effects of pitch ratio in tube banks on its passive acoustics properties which are experimentally investigated. The flow duct experimentation is performed with and without flow to analyse the passive acoustic properties of tube banks. The Experiment is conducted for both the In-line and Staggered tube banks with pitch ratios from 1.2 to 2.5. Experimental results consist of scattering matrix coefficients and power balance. The results are compared to see the pitch ratio effect.