Effect of Condensation on Performance and Design of Extended Surfaces (original) (raw)
Related papers
Condensation Heat Transfer on Enhanced Surface Tubes: Experimental Results and Predictive Theory
Journal of Heat Transfer, 2002
Condensation heat transfer in a bundle of horizontal enhanced surface copper tubes (Gewa C+ tubes) has been experimentally investigated, and a comparison with trapezoidal shaped fin tubes with several fin spacing has been made. These tubes have a specific surface three-dimensional geometry (notched fins) and the fluids used are either pure refrigerant (HFC134a) or binary mixtures of refrigerants (HFC23/HFC134a). For the pure fluid and a Gewa C+ single tube, the results were analyzed with a specifically developed model, taking into account both gravity and surface tension effects. For the bundle and for a pure fluid, the inundation of the lowest tubes has a strong effect on the Gewa C+ tube performances contrary to the finned tubes. For the mixture, the heat transfer coefficient decreases dramatically for the Gewa C+ tube.
Condensation heat transfer coefficients of enhanced tubes.
In solar power generating plants, dry cooling towers are used when there is scarcity of water. Normally, condensation of the steam occurs in dry cooling towers in tubes at inclined angles. Almost all the previous work on condensation was in horizontal and vertical tubes until recently when work was done on condensation in inclined tubes but limited to smooth tubes and one type of enhanced tube. The purpose of this paper is to continue on previous work and present heat transfer coefficients and pressure drops during the condensation of R134a in an enhanced tube of inner diameter of 8.67mm with 60 fins with height of 0.22mm spiraled at an angle of 37 o. The experiments were conducted at condensing temperatures of 30 o C and 40 o C at mass fluxes between 300 kg/m 2 s and 400 kg/m 2 s and various vapour qualities. It was found that the heat transfer coefficients and pressure drops increased with mean quality. Overall, the heat transfer enhancement factors were between 2.1 and 2.9 and the pressure drop penalty factors were between 1.2 and 1.8 with the enhancement more pronounced at lower mass fluxes. Finally, the heat transfer and pressure drops increased with decrease in condensing temperature.
Free-convection condensation on single horizontal pin-fin tubes
Free-Convection Condensation on Pin-Fin Tubes New experimental data are reported for free-convection condensation of ethylene glycol and R-113 on three-dimensional pin-fin tubes. Effects of pin geometry and tube thermal conductivity (for copper, brass and bronze giving a mean range of 400, 120 and 80 W/m K over the range of temperature of interest) were investigated. All tests were performed at near atmospheric pressure with downward flowing vapour at low velocity. Heat-transfer enhancement was found to be approximately twice the corresponding active surface area of the tubes, i.e. the surface area of the parts of the tube and pin surface not covered by condensate retained by surface tension. For ethylene glycol, the best performing pin-fin tube gave a heat-transfer enhancement of 5.8, about 24 % higher than the ‘equivalent’ two-dimensional integral-fin tube (i.e. with the same finroot diameter, longitudinal fin spacing and thickness and fin height). For R-113, the best enhancement was 5.9, about 10 % higher than the equivalent integral-fin tube.
Mathematical model of condensation on the outer surface of tubes with longitudinal fins
Chemical and Petroleum Engineering, 2006
A mathematical model of condensation on finned or ribbed vertical surfaces of various geometry, including the Gregorig, where heat is dissipated in the boiling zone (i.e., during heat exchange in condensers-evaporators) is proposed. The model makes it possible to determine the coefficients of convective and radiative heat transfer with allowance for the temperature and film thickness of the condensate that varies over the height of the surface. Analyzing mathematical descriptions of condensation on the outer finned surfaces of the tubes in condensers-evaporators [1-3], it is possible to isolate the following two types of computational models: 1) the film thickness of the condensate is assumed constant; 2) the film thickness varies, but is independent of the geometry of the condensing surfaces. Both of these types of mathematical models do not make it possible to develop and optimize modern effective condensing surfaces, since they do not reflect the influence exerted by surface geometry on the distribution of the condensate film, and, accordingly, the coefficient of convective heat transfer. Considering familiar results of [2, 3] in which attempts are made to describe condensation on a finned surface, we have developed a new mathematical model that permits calculation of the film thickness of condensate and the coefficient of convective heat transfer for virtually any type of finned or ribbed surface where heat is exchanged in a condensation-boiling system with an accuracy sufficient of engineering practice. Let us examine the distribution of condensate film on vertical surfaces with different fin geometries (Fig. 1). Basic geometric characteristics of a wetted condensing surface exposed to a laminar flow are as follows: r 0-finning interval; b-height of fin; t-width of fin; e-thickness of condensate at bottom of groove; and ε-deflection of liquid surface at phase interface. In [2, 3], the condensing surface is partitioned into three regions (see Fig. 1): I-region of condensation on the terminal section of the fin; II-basic effective region of condensation; and III-region of the drainage groove. Each region is assigned its own law governing film distribution, and the instantaneous flow of mixture through the cross section is determined as the sum of the flows through the three regions. This approach is not entirely valid, since it is not possible to derive a smooth continuous function for the condensing surface, and is associated with a large volume of calculations. Proceeding from the fact that the phase interface of an actual physical system subject to a developed steady-state laminar flow is always a smooth continuous function, the model proposed treats it as a three-dimensional cylinder with a generatrix in the form of a shape function. Here, the shape function for the interface is described by two conjugate polynomials, the exponent and coefficients of which are determined for each geometric fin configuration. In conformity with the approach in question, region I (terminal section) and region II (basic region of condensation) are combined into one. The model therefore consists of two regions-the effective region of the fin and the region of the drainage groove.
2015
An experimental investigation was performed to evaluate condensation and evaporation heat transfer on the outside of a smooth tube, herringbone micro fin tube and the Vipertex 1EHT enhanced heat transfer tube as a function of mass flux. Heat transfer enhancement is an important factor in obtaining energy efficiency improvements in two phase heat transfer applications. Utilization of enhanced heat transfer tubes is an effective enhancement method that is utilized in the development of high performance thermal systems. VipertexTM enhanced surfaces have been designed and produced through material surface modifications, creating flow optimized heat transfer tubes that increase heat transfer. Heat transfer processes that involve phase-change processes are typically efficient modes of heat transfer; however current energy demands and the desire to increase efficiencies of systems have prompted the development of enhanced heat transfer surfaces that can be used in processes involving evapo...
An experimental investigation was conducted into the heat transfer characteristics during in-tube condensation of horizontal smooth, micro-fin, and herringbone tubes. The study focused on the heat transfer coefficients of refrigerants R-22, R-134a, and R-407C inside a series of typical horizontal smooth, micro-fin, and herringbone tubes at a representative average saturation temperature of 40° C. Mass fluxes ranged from 300 to 800 kg/ m 2 s, and vapor qualities ranged from 0.85 to 0.95 at condenser inlet, to 0.05 to 0.15 at condenser outlet. The herringbone tube results were compared with the smooth and micro-fin tube results. The average increase in the heat transfer coefficient of the her-ringbone tube, when compared with the smooth tube at comparable conditions, was found to be 322%, with maximum values reaching 336%. When compared with the micro-fin tube, the average increase in heat transfer coefficient was found to be 196%, with maximum values reaching 215%. Moreover, a new correlation was developed to predict the heat transfer coefficients in a herringbone and micro-fin tube. Semi-local heat transfer coefficients were calculated from the modified Wilson plot technique, using measurements of condenser subsection inlets and outlets, and from knowledge of the temperature gradient on the annulus side. The correlation predicted the semi-local heat transfer coefficients accurately, with 96% and 89% of the data points falling in the ±20% region for the herringbone tube and the micro-fin tube, respectively. The average heat transfer coefficients were accurately predicted, too, with all the data points for the her-ringbone tube and 83% of the data points for the micro-fin tube falling in the ±20% region. The derived heat transfer correlations can be used for design, especially for reversible heat pumps. This research proves that predicting the flow pattern during intermittent and annular flow is not a prerequisite for predicting the heat transfer accurately to within 20% of the measurements.
Comprehensive review of pure vapour condensation outside of horizontal smooth tubes
Nuclear Engineering and Design, 2019
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