Finned Tube Bundles Research Articles

Heat load capability matching principle and its applications to anti-freezing of air-cooled condenser

Air-cooled condenser in power plants takes a risk of freezing in extremely cold days, so it is of benefit to the safe and economical operation of direct dry cooling system to propose the anti-freezing principles and take measures. On the basis of the heat load balance between the exhaust steam and cooling air, the heat load capacity matching principle for the anti-freezing of air-cooled condenser is proposed with reference to the freezing point of water. By applying heat exchanger model to the finned tube bundles of air-cooled condenser, the thermo-aerodynamic behavior of cooling air, the condensation of exhaust steam and the sensible heat rejection of condensate in a representative air-cooled condenser cell are synchronously modeled and resolved. The correlations among the ambient temperature, flow rate of cooling air, exhaust steam flow rate and quality, and back pressure of turbine that prevent the air-cooled condenser from freezing are discussed, and the anti-freezing flow rate of exhaust steam, back pressure of turbine and flow rate of axial flow fan are obtained. The results show that the anti-freezing flow rate of exhaust steam and back pressure both increase with decreasing the ambient temperature and increasing the flow rate of axial flow fan, from which derive the secure steam flow rate that can reduce the back pressure as much as possible to improve thermal efficiency. The anti-freezing fan flow rate increases with increasing the exhaust steam flow rate and ambient temperature, but varies little with back pressure. The increased steam quality will result in a higher heat load at the steam side, which allows a higher rotational speed of fan to be free of freezing for air-cooled condenser. The application of heat load capacity matching principle to the anti-freezing of air-cooled condenser contributes to the secure and optimal operation of dry cooling system in power plants.

POD based modeling on flow and heat transfer of air-cooled condenser influenced by natural wind

Numerous factors, including the meteorological and the geographical conditions as well as the configurations of the fin tube bundles, can affect the performance of an air-cooled condenser (ACC) in a power generating unit. In this work a new model is presented to rapidly and accurately predict the flow and heat transfer characteristics of an ACC under the influence of natural wind. This reduced-order model is based on proper orthogonal decomposition (POD) of the three-dimensional velocity and temperature fields and encompasses natural wind directions ranging from 0° to 90°. In the process of operation in the present paper, a combined procedure involving spare and complementary POD is formulated to treat the issue of high nonlinearity associated with the large variable gradients which generate vortices around the ACC. A flux matching procedure is also introduced for estimation of the weight coefficients of the POD modes. The velocity and temperature fields of four typical test cases were estimated with maximum relative errors of 1.75% and 0.19%, respectively. At the same time, the degree of freedom was reduced by 105 compared to calculations made using computational fluid dynamics. The present research may provide a new and more reliable approach to real-time operational control of ACCs in power plants under polytropic natural wind conditions.

Universal relations for calculation of the drag of transversely finned tube bundles

The drag of cross flow staggered and in-line helically and annularly finned tube bundles is investigated experimentally. The widest range of the reported geometric characteristics of finned tubes and tube bundles is considered. It is shown that when generalizing the experimental data obtained the best result is achieved by taking into account the Reynolds number exponent in the similarity equation as a function of geometric characteristics. Here as dimensionless parameters the geometric similarity of the reduced extended surface and the transverse-to-longitudinal tube pitch ratio are used, and also as the determining dimension in the Reynolds number – the equivalent diameter of the minimum flow section of the tube bundle. The results obtained by this approach describe with a sufficient accuracy 90% of all reported experimental data.

Pumping Power Minimization in Staggered Finned Circular and Elliptic-Tube Heat Exchangers in Turbulent Flow

An experimental study aimed at minimizing pumping power required to supply air through a finned tube bundle configuration is presented in this article. Results were obtained for the Reynolds number based on the smaller ellipse axis (RE2b) ranging from 2,650 to 10,600, i.e., in turbulent flow. In the turbulent regime, pressure drops are expected to vary with eccentricity. Experimental optimization results for finned circular and elliptic tubes show that pumping power can be minimized with respect to tube spacing and eccentricity. In comparison with values obtained for circular tubes, the optimal elliptical arrangements show pumping power reductions from 5% to 10%.

Simulation of air pressure losses in the finned tube bundle of an air condenser

Simulation of air pressure losses in the finned tube bundle of an air condenser

Condensation heat transfer on single horizontal smooth and finned tubes and tube bundles for R134a and propane

For the refrigerants 1,1,1,2-tetrafluoroethane (R134a) and propane (R290), the condensation heat transfer was investigated on coated and uncoated horizontal smooth, standard finned and high performance tubes by experiments and computational fluid dynamics (CFD)-simulations. Measurements were carried out at a saturation temperature of 37°C varying the heat flux between 4kWm−2 and 102 kWm−2. In comparison to the Nußelt theory, enhancement factors of 12.8 to 30.2 were experimentally found for single standard finned and single high performance tubes. For both refrigerants, the high performance tubes showed a larger bundle effect than the standard finned tubes, although the latter show a lower heat transfer performance. In general, experimental results for coated tubes show a slightly lower heat transfer performance and no improvements of the condensate drainage could be observed for the modification. CFD-simulations were used to predict the condensation heat transfer on single tubes for the first time. Our experimental data and CFD-simulations were compared with analytical models available in the literature.

Reduced order analysis of flow and heat transfer for air-cooled condenser of power generating unit

The flow and heat transfer exhibit complex non-linear characteristics for the air-cooled condenser (ACC) of power generating unit because numerous factors, including the meteorological and the geographic conditions, as well as the configurations of wavy finned tube bundles, can affect its performances. In order to quickly and accurately predict the air side velocity and temperature fields of ACC, the reduced order models (ROMs) based proper orthogonal decomposition (POD) method were established, by which the previous 105 DOF in CFD model was reduced to 101 DOF for new case prediction .The weight coefficients for POD modes were obtained by cubic spline interpolation and flux matching procedure (FMP). The air flow fields and correlating temperature fields influenced by environmental natural winds were revealed. It is found that accuracies of the POD solution with cubic spline interpolation are better than that of FMP. However, the FMP procedure, which can extract flow and thermal information from fewer observations, has better robustness than interpolation method. Hence, it is more appropriate for the POD solution of extrapolated cases. The present research may provide a rapid and reliable approach for the optimization of real-time operation of air-cooled power generating units.

Effects of ambient winds on the thermo-flow performances of indirect dry cooling system in a power plant

Ambient winds are one of the key issues in the design and operation of indirect dry cooling system in a power plant. It is of benefit to the optimization of air-cooled heat exchangers and dry-cooling towers to investigate the impacts of ambient winds on the thermo-flow performances of indirect dry cooling system. Based on a representative 4 × 660 MW dry cooling power plant with the flue gas desulfurization and stack installed inside the dry-cooling tower, the physical and mathematical models of the air-side fluid and heat flows at various wind speeds and in various wind directions are developed. The velocity, pressure and temperature fields are presented, and the flow rate and heat rejection for the air-cooled heat exchangers and cooling towers are obtained by using CFD simulations. The results show that the fluid and heat flows of indirect dry cooling system are dominated by the wind induced and buoyancy driven air flows, which are the results of balance between the inertial force, and pressure, viscous and buoyancy forces. In the absence of ambient winds, the hot plume discharge experiences a reverse pressure gradient. The thermo-flow performances of indirect dry cooling system are deteriorated with increasing the wind speed at low wind speeds, but get improved at high wind speeds despite of the hot air penetration through parts of the air-cooled heat exchanger. The frontal finned tube bundles of air-cooled heat exchanger are superior to other finned tube bundles in thermo-flow performances. It will contribute to the proposal of measures against the adverse impacts of ambient winds for the indirect dry cooling system in a power plant.

Fluidelastic Instability in Normal and Parallel Triangular Arrays of Finned Tubes

An experimental study was carried out to investigate fluidelastic instability in finned tube bundles in normal and parallel triangular arrays. Three arrays of each geometry type were studied experimentally: two arrays with serrated, helically wound finned tubes of different fin densities, and a bare tube array with the same base diameter as the finned tubes. All six tube arrays studied had the same tube pitch. The finned tubes under consideration were commercial finned tubes typically used in the fossil and process industries. For the purpose of the present investigation, the concept of “effective diameter” of a finned tube, as previously used to predict vortex shedding, was used to compare the finned tube results with other finned tube results as well as the existing bare tube world data. The experimental results for the triangular arrays show that the fin’s structure strongly influences the fluidelastic stability of finned tube bundles and the fin pitch is demonstrated to reduce the difference in the stability threshold between the tube array geometries as the fin density increases. Overall, the effect of serrated fins on fluidelastic instability is very complex and array geometry dependent, stabilizing some arrays and destabilizing others. Clearly, the effect of fins cannot be accounted for by the simple use of an effective diameter of an equivalent bare tube. An earlier version of this paper appeared at the ASME 2010 FSI Conference, FEDSM-ICNMM2010-30223.

Experimental Investigation of Heat Transfer and Flowing Resistance for Air Flow Cross over Spiral Finned Tube Heat Exchanger

In order to study heat transfer and flowing resistance characteristics for air flow cross over spiral finned tube heat exchanger, 13 spiral finned tube bundles heat exchangers are experimental investigated, the specimens have constant outer diameter of base tube (d=32mm), different fin pitch (t/d=0.22∼0.5), fin height (h/d=0.22∼0.5), transverse tube pitch (S1/d=2∼3.3125), longitudinal tube pitch (S2/d=2∼3.3125), the experiments are done within the range of fluid flowing Re number (Re=5×103∼5.5×104). Experiments obtain heat transfer Nusselt number correlation and flowing resistance Euler number correlation with fluid flowing Re number, fin pitch t, fin height h, transverse tube pitch S1, and longitudinal tube pitch S2.

Improvement of heat transfer with perforated circular holes in finned tubes of air-cooled heat exchanger

The present study investigated the effect of perforated circular finned-tube (PCFT) on the convective heat transfer performance of circular finned-tube heat exchangers. The air-side convective heat transfer coefficients increased by 3.55% and 3.31% for 2-hole and 4-hole PCFT cases, respectively. The increase in the convective heat transfer coefficient was related to the reduction of the recirculation region by introducing the perforations at the flow-separation locations on the finned tube. The pressure drop across the finned-tube bundles increased by 0.68% and 2.08% for the 2-hole PCFT and 4-hole PCFT cases, respectively. The greater pressure drop in the case of the 4-hole PCFT might be due to excessive flow disturbances produced by multiple perforations. The fin factor defined as the ratio of the % increase of the convective heat transfer coefficient and that of the pressure drop was 5.19 for the 2-hole PCFT case, whereas that was 1.59 for the 4-hole PCFT case.

Thermo-flow characteristics and air flow field leading of the air-cooled condenser cell in a power plant

Special A-frame geometry of the air-cooled condenser cell and the complicated flow field at the exit of the axial flow fan bring on the air mal-distribution on the surface of the finned tube bundles and the deteriorated thermo-flow performances of a condenser cell. It is of benefit to the design and operation optimization of the direct dry cooling system in a power plant to investigate the thermo-flow characteristics of the condenser cell and propose the flow leading measures of cooling air. On the basis of the representative configuration of the air-cooled condenser cell in a 600 MW direct dry cooling power plant, the computational models of the air side fluid and heat flows are built, in which the actual fan blade geometric details are considered. Various flow field leading ways of cooling air are presented and the thermo-flow characteristics in the A-frame condenser cell and through the finned tube bundles are compared. Results show that the flow field leading measures can result in the increased volumetric flow rate and heat rejection, thus bringing on the improved performance of the condenser cell. The improvement of thermo-flow performances depends upon the geometric details of the flow guiding device.

Study of Heat Storage in Shallow Fluidized Beds

This paper presents the theoretical analysis and experimental study of the performance of a shallow fluidized bed as a storage system for thermal energy. River sand is used as a bed with different height values and compressed heated air as an external fluidizing. A bundle of finned tubes is used as a heat exchanger immersed in the bed for the extraction of stored energy from the bed. Empirical correlations are offered by experimental work, which show the relations between the instantaneous local temperature within the bed and time as a function of both bed height and inlet air velocity. Keywords: Heat, Transfer, Fluidized Beds, Heat Storage,.

An asymptotic approach to generalizing the experimental data on convective heat transfer of tube bundles in crossflow

Based on the analysis of experimental data the universal methods of calculating convective heat transfer of smooth and finned tube bundles in the crossflow have been developed over the ranges of geometric characteristics covering all practical needs at the Reynolds number Re = 3 × 10 3…1 × 10 5. The distinctive feature of the methods proposed is that the generalized similarity equation of convective heat transfer takes into account the dependence of the Reynolds number exponent on tube pitch characteristics in a bundle. This has allowed the mechanism of heat transfer and hydraulic performance in tube bundles to be taken into utmost consideration and the asymptotic character to be given to the generalized dependence. In turn, this has permitted one to show the presence of maximum of heat transfer intensity and also to cover limiting combinations of pitches, at which differences in staggered and in-line arrangements of tubes are leveled, i.e., practically the restrictions on the ranges of tube pitch characteristics of bundles can be removed.

Space characteristics of the thermal performance for air-cooled condensers at ambient winds

Ambient winds may lead to poor fan performance, exhaust air recirculation and mal-distribution of the air across the tube bundles of the air-cooled condensers in a power plant. Investigations of the impacts of the ambient winds on the air-cooled condensers are key area of focus. Based on a representative 2 × 600 MW direct dry cooling power plant, the physical and mathematical models of the air-side fluid and heat flow in the air-cooled condensers at various ambient wind speeds and directions are set up by introducing the radiator model to the fin-tube bundles. The volumetric flow rate, inlet air temperature and heat rejection for different air-cooled condensers as a whole, condenser cells and fin-tube bundles are obtained by using CFD simulation. The results show that the thermo-flow performances for the air-cooled condenser as a whole, condenser cells and heat exchanger bundles vary widely in space. The thermal performances of the air-cooled condensers, condenser cells and fin-tube bundles at the downstream are generally superior to those at the upwind. It is of use for the upwind fan regulations and the A-frame condenser cell geometric optimization to investigate the space characteristics of the thermal performance for the air-cooled condensers in a power plant.

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85°C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.

Performance monitoring of direct air-cooled power generating unit with infrared thermography

Abstract Infrared thermography technique was employed to monitor overall as well as local surface temperature distributions of air-cooled condensers (ACC) in order to investigate the influence of ambient air temperatures, surface fouling of fin-tube bundles and natural wind on performance of air-cooled power generating units. Three typical direct air-cooled power generating units in operation in north China were selected as the research objects, which are 300 MW sub-critical units with ACC of three-row fin-tube bundles, 600 MW sub-critical unit and 600 MW supercritical unit with ACC of single-row fin-tube bundle respectively. The monitoring results revealed that the counterflow ACC units were vulnerable to freezing in winter. The results also cast light on the uneven steam flow distributions inside fin-tubes of different parallel-flow ACC units, especially when the redundant cooling areas of ACC Island were large. The characteristics of surface temperature distributions were obtained in different regions of ACC units under surface fouling conditions. It was also acquired that the gentle natural wind can have an impact on the heat transfer of most cooling areas of ACC island by forming overhead vortex. This should be paid attention to as well as besides the effect of thermal recirculation under large natural wind conditions. The present study may benefit the optimal operation of air-cooled power generating units.

Inverse determination of the heat transfer characteristics on a circular plane fin in a finned-tube bundle

In this work, we present the numerical results of the average heat transfer coefficients, \( \overline{{h_{\varphi } }} \), over a circular plane fin in a finned-tube bundle for both aligned and staggered arrangements as well as the fin efficiency and the heat flux dissipated from the whole fin. The study covers a wide range of Reynolds number (2 × 103–3 × 104), for three different positions of the finned tube inside the heat exchanger. The temperature distribution on the fins surfaces was obtained experimentally using infrared thermography technique. The predicted values of the heat transfer coefficient were obtained numerically using the finite element method in conjunction with the conjugate gradient algorithm and the measured temperatures.

Experimental investigation of heat transfer and pressure drop in serrated-fin tube bundles with staggered tube layouts

An experimental investigation of the heat transfer and pressure drop performance of ten finned tube bundles using serrated fins is presented. All tube bundles had staggered layouts, and the influence on varying tube bundle layout, tube and fin parameters are presented. The heat transfer coefficient experienced a maximum when the flow areas in the transversal and diagonal planes were equal. An increase in the fin pitch increased the heat transfer coefficient; the same was observed with an increase in fin height. The pressure drop coefficient showed no influence of the tube bundle layout for small pitch ratios, but dropped significantly for higher ratios. Increasing fin pitch reduced the pressure drop, whereas varying fin height had insignificant effect. None of the literature correlations were able to reproduce the experiments for the entire range of tested conditions. A set of correlations were developed, reproducing the experimental data to within ±5% at a confidence interval of 95%.

Porous Medium Modeling of Air-Cooled Condensers

This article presents a porous media transport approach to model the performance of an air-cooled condenser. The finned tube bundles in the condenser are represented by a porous matrix, which is defined by its porosity, permeability, and the form drag coefficient. The porosity is equal to the tube bundle volumetric void fraction and the permeability is calculated by using the Karman–Cozney correlation. The drag coefficient is found to be a function of the porosity, with little sensitivity to the way this porosity is achieved, i.e., with different fin size or spacing. The functional form was established by analyzing a relatively wide range of tube bundle size and topologies. For each individual tube bundle configuration, the drag coefficient was selected by trial and error so as to make the pressure drop from the porous medium approach match the pressure drop calculated by the heat exchanger design software ASPEN B-JAC. The latter is a well-established commercial heat exchanger design program that calculates the pressure drop by using empirical formulae based on the tube bundle properties. A close correlation is found between the form drag coefficient and the porosity with the drag coefficient decreasing with increasing porosity. A second order polynomial is found to be adequate to represent this relationship. Heat transfer and second law (of thermodynamics) performance of the system has also been investigated. The volume-averaged thermal energy equation is able to accurately predict the hot spots. It has also been observed that the average dimensionless wall temperature is a parabolic function of the form drag coefficient. The results are found to be in good agreement with those available in the open literature.