Experimental studies of a double-pipe helical heat exchanger

An experimental study of a double-pipe helical heat exchanger was performed. Two heat exchanger sizes and both parallel flow and counterflow configurations were tested. Flow rates in the inner tube and in the annulus were varied and temperature data recorded. Overall heat transfer coefficients were calculated and heat transfer coefficients in the inner tube and the annulus were determined using Wilson plots. Nusselt numbers were calculated for the inner tube and the annulus. The inner Nusselt number was compared to the literature values. Though the boundary conditions were different, a reasonable comparison was found. The Nusselt number in the annulus was compared to the numerical data. The experimental data fit well with the numerical for the larger heat exchanger. But, there were some differences between the numerical and experimental data for the smaller coil; however these differences may have been due to the nature of the Wilson plots. Overall, for the most part the results confirmed the validation of previous numerical work.     

Determination of heat transfer correlations for plate-fin-and-tube heat exchangers

This paper presents a numerical method for determining heat transfer coefficients in cross-flow heat exchangers with extended heat exchange surfaces. Coefficients in the correlations defining heat transfer on the liquid- and air-side were determined using a non-linear regression method. Correlation coefficients were determined from the condition that the sum of squared fluid temperature differences at the heat exchanger outlet, obtained by measurements and those calculated, achieved minimum. Minimum of the sum of the squares was found using the Levenberg-Marquardt method. The outlet temperature of the fluid leaving the heat exchanger was calculated using the mathematical model describing the heat transfer in the heat exchanger. Since the conditions at the liquid-side and those at the air-side are identified simultaneously, the derived correlations are valid in a wide range of flow rate changes of the air and liquid. This is especially important for partial loads of the exchanger, when the heat transfer rate is lower than the nominal load. The correlation for the average heat transfer coefficient on the air-side based on the experimental data was compared with the correlation obtained from numerical simulation of 3D fluid and heat flow, performed by means of the commercially available CFD code. The numerical predictions are in good agreement with the experimental data.     

Experimental determination of correlations for average heat transfer coefficients in heat exchangers on both fluid sides

This paper presents an experimental–numerical method for determining heat transfer coefficients in cross-flow heat exchangers with extended heat exchange surfaces. Coefficients in the correlations defining heat transfer on the liquid- and air-side were determined based on experimental data using a non-linear regression method. Correlation coefficients were determined from the condition that the weighted sum of squared liquid and air temperature differences at the heat exchanger outlet, obtained by measurements and those calculated, achieved minimum. Minimum of the sum of the squares was found using the Levenberg–Marquardt method. The uncertainty in estimated parameters was determined using the error propagation rule by Gauss. The outlet temperature of the liquid and air leaving the heat exchanger was calculated using an analytical model of the heat exchanger.     

Heat transfer characteristics of a new helically coiled crimped spiral finned tube heat exchanger

In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil. Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate. The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing inlet water temperature.     

Visualization and determination of local heat transfer coefficients in shell-and-tube heat exchangers for in-line tube arrangement by mass transfer measurements

The local heat transfer coefficients on the shell-side of shell-and-tube heat exchangers for in-line tube arrangement are visualized and determined from mass transfer measurements. The mass transfer experiments are carried out using a technique based on absorption, chemical and coupled colour reaction. Local mass transfer coefficients are measured for fully developed flow conditions on each tube surface. These coefficients were transformed to heat transfer coefficients by employing the analogy between heat and mass transfer. The averaged heat transfer coefficients and the pressure drop are compared with the predictions from the literature. Received on 2 May 1997     

The optimum fin spacing of circular tube bank fin heat exchanger with vortex generators

In real application, once the pattern of fin is determined, fin spacing of tube bank fin heat exchanger can be adjusted in a small region, and air flow velocity in the front of the heat exchanger is not all the same. Therefore, the effects of fin spacing on heat transfer performance of such heat exchanger are needed. This paper numerically studied the optimal fin spacing regarding the different front flow velocities of a circular tube bank fin heat exchanger with vortex generators. To screen the optimal fin spacing, an appropriate evaluation criterion JF was used. The results show that when front velocity is 1.75 m/s, the optimal fin spacing is 2.25 mm, when front velocity is 2.5 m/s, the optimal fin spacing is 2 mm, and when front velocity is higher than 2.5 m/s, the optimal fin spacing is 1.75 mm.     

Thermo-hydraulic characterization of a louvered fin and flat tube heat exchanger

In the present study, a whole heat exchanger with a hydraulic diameter of 2.3 mm is tested, which is a minichannel heat exchanger according to the Kandlikar classification. This is a louvered fin and flat tube heat exchanger currently used in car cooling systems, also known as radiator. A glycol-water mixture (60/40 in volume) circulates through the tubes at flows ranging from 100 to 7800 l/h and at a supply temperature of 90 °C. This fluid is cooled with ambient air at a temperature of 20 °C and at frontal air velocities varying between 0.5 and 7 m/s. The thermohydraulic performance of the heat exchanger is compared with the classical correlations given in the literature for the heat transfer and the friction factor calculation. On the glycol-water side the heat exchanger is characterized for Reynolds numbers from 30 to 8000. A first comparison is carried out with the correlations available in the literature with a purely predictive model by obtaining a predictive value with a systematic under prediction lower than 10%. In a second step a semi-empirical model is considered to identify the experimental heat transfer coefficients for this application.     

Analytical solution of conjugate heat transfer and optimum configurations of flat-plate heat exchangers with circular flow channels

An analytical solution is developed for conjugate heat transfer in a flat-plate heat exchanger with circular embedded channels. The analysis was carried out for fully-developed conditions in the circular tube and uniform heat flux at the plate boundary. The results are applicable to cooling channels that are 50 μm or more in diameter with a large length–diameter ratio. The thermal characteristics of the heat exchanger have been examined for a wide range of the relevant independent parameters and optimum designs for three different sets of constraints have been presented. It was found that the overall thermal resistance increases with the depth of the tube from the heated surface, as well as the spacing between the tubes. For a given combination of tubes’ depth and spacing, there is a certain tube diameter at which the thermal resistance attains a minimum value.     

Flow boiling heat transfer of R134a in the multiport minichannel heat exchangers

The flow boiling heat transfer characteristics of R134a in the multiport minichannel heat exchangers are presented. The heat exchanger was designed as the counter flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Two inner tubes were made from extruded multiport aluminium with the internal hydraulic diameter of 1.1 mm for 14 numbers of channels and 1.2 mm for eight numbers of channels. The outer surface areas of two inner test sections are 5979 mm² and 6171 m², while the inner surface areas are 13,545 mm² and 8856 mm² for 14 and eight numbers of channels, respectively. The outer tube of heat exchanger was made from circular acrylic tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at the heat fluxes between 15 and 65 kW/m², mass flux of refrigerant between 300 and 800 kg/m² s and saturation pressure ranging from 4 to 6 bar. For instance the boiling curve, average heat transfer coefficients are discussed. The comparison results of two test sections with different the number of channels are investigated. The results are also compared with nine existing correlations. The new correlation for predicting the heat transfer coefficient was also proposed.     

Abhängigkeit der Wärmeübergangskoeffizienten vom Wandtemperaturverlaufund die Konsequenzen für die Berechnung von Wärmedurchgangskoeffizienten bei Wärmetauschern

In a recent article [2] it has been shown that the common equation in VDI Heat Atlas [1], which has been used for the calculation of overall heat transfer coefficients, satisfies the energy balance of the heat exchanger in seldom exceptional cases only. This equation has been corrected for constant wall temperature. In this article an equation for the calculation of overall heat transfer coefficients valid for any shape of the wall temperature will be developed at which it becomes obvious, that only in the case of identical shapes of wall temperature in measurement and real heat exchanger process the calculation according to VDI Heat Atlas [1] is valid. In all other cases this procedure leads to errors.     

Pressure drop and heat transfer comparison for both microfin tube and twisted-tape inserts in laminar flow

Experiments were carried out to compare pressure drop and heat transfer coefficients for a plain, microfin, and twisted-tape insert-tubes. The twisted-tape experiments include three different twist ratios each with two different widths. The data were taken at Reynolds numbers well in the laminar region. The heat transfer data were obtained in a single shell-and-tube heat exchanger where steam is used as a heat source to obtain a uniform wall temperature and the working fluid in the tube is oil. The twist ratio and the width of the tape seem to have a large effect on the performance of the twisted-tape insert. The results demonstrate that as the twist ratio decreases, the twisted-tape will give better heat transfer enhancement. The loose-fit (W=10.8 mm) is recommended to be used in the design of heat exchanger where low twist ratios (Y=5.4, and Y=3.6) and high pressure drop situations are expected since it is easier to install and remove for cleaning purposes. Other than these situations, the tight-fit tape gives a better performance over the loose-fit tape. For the microfin tube tested in this paper, the data shows a small increase in both heat transfer and pressure drop. This type of microfin tube is not recommended to be used in laminar flow conditions.     

Experimental investigation on condensation heat transfer and pressure drop of R134a in a plate heat exchanger

Condensation heat transfer of R134a in a vertical plate heat exchanger was investigated experimentally. The local heat transfer coefficients are determined by means of the measured local wall temperatures. A differential energy balance model is developed for data evaluation. It is found that the correlation proposed by Shah using Ψ and Z factors is suitable for condensation in plate heat exchangers and is adopted to fit the measured data.     

The influence of oil on nucleate pool boiling heat transfer

The influence of various oil contents in R134a is investigated for nucleate pool boiling on copper tubes either sandblasted or with enhanced heating surfaces (GEWA-B tube). Polyolester oils (POE) (Reniso Triton) with medium viscosity 55 cSt (SE55) and high viscosity 170 cSt (SE170) were used. Heat transfer coefficients were obtained for boiling temperatures between −28.6 and +20.1°C. The oil content varied from 0 to 5% mass fraction. For the sandblasted tube and the SE55 oil the heat transfer coefficients for the refrigerant/oil-mixture can be higher or lower than those for the pure refrigerant, depending on oil mass fraction, boiling temperature and heat flux. In some cases the highest heat transfer coefficients were obtained at a mass fraction of 3%. For the 170 cSt oil there is a clear decrease in heat transfer for all variations except for a heat flux 4,000 W/m² and −10.1°C at 0.5% oil content. The heat transfer coefficients are compared to those in the literature for a smooth stainless steel tube and a platinum wire. For the enhanced tube and 55 cSt oil the heat transfer coefficients are clearly below those for pure refrigerant in all cases. The experimental results for the sandblasted tube are compared with the correlation by Jensen and Jackman. The calculated values are within +20 and −40% for the medium viscosity oil and between +50% and −40% for the high viscosity oil. A correlation for predicting oil-degradation effects on enhanced surfaces does not exist.     

Numerical estimation of mixed convection heat transfer in vertical helically coiled tube heat exchangers

A numerical investigation of the mixed convection heat transfer from vertical helically coiled tubes in a cylindrical shell at various Reynolds and Rayleigh numbers, various coil‐to‐tube diameter ratios and non‐dimensional coil pitches was carried out. The particular difference in this study compared with other similar studies is the boundary conditions for the helical coil. Most studies focus on constant wall temperature or constant heat flux, whereas in this study it was a fluid‐to‐fluid heat exchanger. The purpose of this article is to assess the influence of the tube diameter, coil pitch and shell‐side mass flow rate on shell‐side heat transfer coefficient of the heat exchanger. Different characteristic lengths were used in the Nusselt number calculations to determine which length best fits the data and finally it has been shown that the normalized length of the shell‐side of the heat exchanger reasonably demonstrates the desired relation. Copyright © 2010 John Wiley & Sons, Ltd.     

The cooling heat transfer characteristics of the supercritical CO2 in micro-fin tube

This study intended to verify the cooling heat transfer characteristics of supercritical gas for refrigerating and air-conditioning devices that use CO₂, a natural refrigerant, as the operating fluid. Experiments were performed with a gas cooler, which was the test part. The gas cooler was a heat exchanger made of a micro-fin tube with an inner diameter of 4.6 mm and an outer diameter of 5.0 mm. The experiment results are summarized as follows. The heat transfer coefficient, according to the mass flux, peaked at the low cooling pressure of 8.0 MPa in the gas cooler, and reached its minimum at the high pressure of 10.0 MPa. Furthermore, when the mass flux of the refrigerant increased, the coefficient increased faster with the lower cooling pressure in the gas cooler. The heat transfer coefficient, according to the shape of the heat transfer tube, showed that the maximum values of the CO₂ cooling heat transfer coefficients of the smooth tube and the micro-fin tube were found at 44.7 °C, which were the pseudo-critical temperatures for the entrance pressures. It was found that the cooling heat transfer coefficient of the micro-fin tube increased by 12–39 % more than that of the smooth tube. The experiment results for the CO₂ heat transfer coefficients of the smooth tube and the micro-fin tube were compared with the results estimated from previous correlations. It was found that the experiment values generally significantly differed from and the experiment values greater than the estimated values. The differences were especially greater in the vicinity of the critical temperature points. Based on these results, a new correlation was suggested that includes the density ratio and the specific heat ratio.     

Sizing of heat exchangers with non-uniform coefficients

A technique is presented to include the effects of a non-uniform, overall heat transfer coefficient in double-pipe heat exchanger analysis using the effectiveness (or efficiency) method. The local overall coefficient is permitted to vary with the local temperature difference between the two fluids according to (T?t)ⁿ, where the exponent “n” assumes individual values for various physical situations. A procedure is provided to estimate the appropriate value of “n” for a particular problem. The development provides a correction factor to the ordinary results of the effectiveness method for uniform overall coefficients.     

Quantitative infrared investigation of local heat transfer in a circular finned tube heat exchanger assembly

This work deals with the local heat transfer coefficient evaluation over the fin of the second row of a staggered circular finned tube heat exchanger assembly. The coefficient distribution is determined by using a transient technique and by calculating the energy balance during the fin cooling. The calculation model takes into account radiation with the surrounding and lateral heat conduction into the material. The method uses infrared measurements and integration between time bounds that depend on space. It is proposed to choose the integration bounds with an original criterion based on local heat transfer. Validation is performed on the reference case consisting in a thin plate in an aerodynamically and thermally developing channel flow. Then, distributions of Nusselt number on the circular fin are presented for several Reynolds numbers. The high resolution of the whole method and set-up allow detecting thermal imprints of developing horseshoe vortices. These imprints are analyzed by following their angular evolution around the tube.     

An improved scheme for determining heat transfer correlations from heat exchanger regression models with three unknowns

A nonlinear regression scheme for determining heat transfer correlations from overall heat exchanger measurements is presented. The proposed scheme is valid for any two-fluid heat exchanger data where only one fluid's thermal resistance varies. The resulting regression model has three unknowns; two are associated with the variable resistance, and the third is the sum of all other resistances. The model can be analyzed only by using nonlinear regression because it cannot be transformed into linear form. Unlike existing approximate methods of analysis (known as modified Wilson plots), the proposed scheme is guaranteed to converge if a solution exists. The scheme was successfully tested by analyzing data collected on a tube-fin heat exchanger with continuous plain fins.     

Convective heat transfer in the laminar–turbulent transition region with molten salt in a circular tube

In order to understand the heat transfer characteristics of molten salt and testify the validity of the well-known empirical convective heat transfer correlations, experimental study on transition convective heat transfer with molten salt in a circular tube was conducted. Molten salt circulations were realized and operated in a specially designed system over 1000 h. The average forced convective heat transfer coefficients of molten salt were determined by least-squares method based on the measured data of flow rates and temperatures. Finally, a heat transfer correlation of transition flow with molten salt in a circular tube was obtained and good agreement was observed between the experimental data of molten salt and the well-known correlations presented by Hausen and Gnielinski, respectively.     

Sizing problem for a cross flow heat exchanger with wing-type vortex generator

In the present study, sizing of a single pass cross flow heat exchanger with unmixed fluid streams has been investigated. The heat exchanger is a cross flow heat exchanger. It has overall dimensions of 20 × 20 × 20 cm. Two the most common heat exchanger design problems are the rating and sizing problem. Sizing problems deal with designing an exchanger and determining its physical size to meet the specified heat duty, pressure drops and other considerations. It means the determination of the exchanger construction type, flow arrangement, heat transfer surface geometries and materials, and the physical sizes of an exchanger to meet specified heat transfer and pressure drop. In this study, the physical size (length, width, height, mass flow rates of both fluids and surface areas on each side of the exchanger) are determined. Inputs to the sizing problem are surface geometries, fluid mass flow rates, inlet and outlet fluid temperatures and pressure drop on each side. Dimensions of L _a , L _b , and L _c for the selected surfaces were investigated such that the design meets the heat duty and pressure drops on both sides exactly.