Rotating Heat Exchanger Research Articles

Performance investigation of a supergravity CO2 refrigeration cycle under off-design conditions

The transcritical CO2 refrigeration cycle is a research hotspot in CO2 refrigeration technology, but it suffers from significant throttling, compression, and gas cooling losses. The recently proposed supergravity CO2 refrigeration cycle, which utilizes centrifugal potential energy-pressure energy conversion in the rotating heat exchanger, provides a new approach to address these issues. However, the flow efficiency of the serpentine rotating channels requires further validation, and the operating characteristics of the system under off-design conditions need exploration. To this end, we first conducted a study on the pressure drop characteristics of a serpentine rotating channel using experimental and numerical simulation methods. We found a strong linear relationship between the friction factor and the rotation number, with the flow efficiency approaching 1.0 beyond a certain threshold, providing critical support for the cycle. Additionally, we established a distributed parameter model for the cycle and theoretically analyzed its operational performance. The results show that the COP of this cycle reaches 6.91 under air conditioning conditions. This is primarily because the local isentropic compression/expansion efficiency of CO2 in the rotating channel exceeds 99 %. Additionally, the temperature slip during the gas cooling process is only 1.2 °C, demonstrating the feasibility of achieving nearly isothermal compression through the rotating heat exchanger. Furthermore, the results showed that changes in boundary conditions resulted in minimal changes in system pressure differential, not exceeding 2 %. However, increases in CO2 charge and rotational speed led to substantial changes in system pressure differential, increasing by 39 % and 51 %, respectively. These trends reflect the effect of the centrifugal potential energy-pressure energy conversion on the operating characteristics of the system. In addition, there exists a maximum charge limit to prevent the cycle from entering the two-phase region. There is also an optimal combination of rotating heat exchanger and compressor speeds that ensures maximum COP while guaranteeing the cooling capacity. The main novelty of this paper lies in elucidating the coupling mechanism between operational parameters and the thermodynamic parameters of the supergravity CO2 refrigeration cycle.

Влияние нестационарности течения теплоносителя в канале матрицы вращающегося теплообменного аппарата на его тепловые параметры при ламинарном течении

The paper studies influence of the non-stationarity process of the laminar gas flow in the band flat matrix channel of the rotating heat exchanger on its thermal characteristics. Conjugate heat transfer was mathematically simulated with the airflow along the steel walls forming the channel under study. In view of peculiarities of the rotating heat exchanger operation and to obtain parameters of the conjugate heat transfer in the nominal mode, mathematical simulation was performed many times using the results previously obtained as the initial conditions for the following calculations. It was established that gas flow in the investigated matrix channel of the heat exchanger became cyclically repetitive after the sixty cycles. Distribution curves of Nusselt criterion local values along the channel length at all time steps of the gas flow process, as well as the Nusselt criterion values averaged over the channel length were obtained. It was shown that the length-averaged Nusselt criterion value was smoothly increasing with time. The time-averaged Nusselt criterion value was determined, and it was somewhat exceeding the values obtained in the stationary setting.

Thermodynamic investigation of a centrifugal reverse Brayton cycle for refrigeration in air conditioning filed

The reverse Brayton cycle (RBC) is known to exhibit a lower coefficient of performance (COP) when compared to conventional vapor-compression refrigeration systems. To tackle this issue, this study proposes a centrifugal reverse Brayton cycle (CRBC) that features a rotating heat exchanger, which utilizes the conversion mechanism between equal potential energy and pressure energy to enhance system performance. The findings indicate that the COP of the CRBC is roughly inversely proportional to the working fluid density, making CO2 a more suitable option for CRBC systems than conventional working fluids. We propose three different cycles for the CRBC system: the diamond cycle, trapezoid cycle, and rectangle cycle. From both technical and operational perspectives, the trapezoid cycle appears to be the preferred choice for CRBC. When the isentropic efficiency of the CRBC system exceeds 95%, its coefficient of performance (COP) ranges from 5.31 to 5.7, whereas the COP of transcritical carbon dioxide refrigeration cycle (tCO2), air RBC, and CO2 RBC are around 4.3, 1.9, and 0.79, respectively. This suggests that the performance of the CRBC system surpasses that of RBC by a significant margin, while achieving comparable performance to conventional vapor compression refrigeration systems. Moreover, the dimensionless power reflecting the installation capacity requirements for CRBC, tCO2, air RBC and CO2 RBC are around 0.2, 0.35, 5.3 and 1.9, respectively. This indicates that the compressor/expander capacity demand for system CRBC approaches the thermodynamic minimum.

Heat exchangers for liquid-free cooling of rotating shafts

Heat reduces the performance of machine tool spindles. In particular, the heating of the shaft leads to numerous undesirable effects. Fluid-based shaft cooling systems exist but are expensive and energy-intensive. For this reason, a novel, liquid-free shaft cooling concept is researched. Heat is transferred from a rotating heat exchanger to a non-rotating and cooled structure through a narrow air gap. In this paper, the cooling potential of this concept is identified simulatively and experimentally by quantifying the heat transfer coefficients as well as the generated air friction. For the first time, a suitable simulation setup for the modelling of such heat exchangers is presented, providing a fundamental contribution to the design of liquid-free cooling systems.

Повышение эффективности охлаждения оборотной воды в испарительных охладителях с псевдоожиженным слоем

The article describes the results from experimental investigations into the hydrodynamics of a three-phase fluidized bed. Calculation dependences of the three-phase fluidized bed height on the spraying density, which can be used for achieving more efficient cooling of circulating water in evaporative coolers, are given. The optimal conditions for stable operation and the technological parameters characterizing the water cooling process in a three-phase fluidized bed are determined. An experimental setup for carrying out full-scale experiments on evaporative cooling of circulating water in a three-phase fluidized bed is constructed, and the actual potential of the evaporative water cooling technology under real conditions is determined. It is shown that the calculated and experimental data are in good agreement with each other, and validity of the accomplished investigations has been confirmed. A mathematical model of the circulating water evaporative cooling process in a three-phase fluidized bed is proposed. Based on the numerical implementation of the proposed model, the dependences of cooled water and spent humid air temperatures at the evaporator outlet on relative humidity ф, spraying ratio ц, air velocity W, and ambient air and inlet water temperatures to and tE1 have been established. An algorithm and a program have been developed for determining the temperature of outlet water tH2 and humid air tm at the outlet of the evaporative cooler. A linear dependence of the circulating water evaporative cooler’s energy coefficient on the spraying coefficient has experimentally been established for the considered cooler type. A dependence of the evaporated moisture fraction in the considered circulating water evaporative cooler on the cooled water temperature at the cooler inlet and on the spraying coefficient is established. An evaporative air cooling method is proposed, which is embodied by using an evaporative chamber with a three-phase fluidized bed and a regenerative rotating heat exchanger.

Comparison of the analytical solutionof efficieтcy regenerative rotating heatexchangethe device with resultsof numericalcalculations

The article is devoted to the thermal-hydraulic calculation of the rotating heat exchanger of a gas turbine engine. More specifically, the purpose of this work is to determine the error of the known analytical calculation of the efficiency of the regenerative heat exchanger in comparison with numerical calculation. Analytical and numerical calculations of the rotating heat exchanger are performed. The main problem of creating a methodology for the analytical solution of processes occurring in rotating heat exchangers is that it is necessary to solve the problem of non-stationary heat transfer from the hot coolant to the coolant through the regenerator packing. The solution of any non-stationary process of heat transfer requires knowledge of the boundary and initial conditions of the process. As initial conditions, it is necessary to use the dependence of the regenerator packing temperature field on the steady state of its operation. Such a relationship can only be obtained after the calculation is completed. In this case, the solution of the heat transfer problem in a rotating heat exchanger is possible using two methodological approaches. The first possibility is a solution by successive approximations. In addition to the fact that such a technique is very time-consuming, it allows us to obtain only a numerical solution of the problem, the possibilities of analysis are limited. The second possibility is the use of one or several assumptions. Such an analytical solution was obtained by Stepanov G.Yu. It is clear that the use of assumptions in analytical analysis or in analytical calculations requires an assessment of the accuracy of the methodology. An estimate of the accuracy of the analytical solution to the efficiency of a rotating heat exchanger is given in the proposed work. It is shown that the error of the analytical solution of the regenerator efficiency in the zone of high regeneration degree values is insignificant in comparison with numerical calculations. Analytical solutions proposed by Stepanov G.Yu., are accessible and effective in the process of creating regenerators. Of particular importance are analytical calculations of the efficiency of regenerators when recording solutions in the form required by the similarity theory. An increase in the error in the analytical calculation of the efficiency of the regenerator is noted with a decrease in the ratio of the water equivalents of the heat-transfer packing of the rotating heat exchanger and coolants (gas and air). Marked increase of error of the analytical calculation of the effectiveness of the Regener-Torah in the decrease of water equivalent heat transfer gaskets. no-return heat exchanger and coolants (gas and air).

Experimental Investigation of a Friction Ventilator

Standard decentralized ventilation systems typically consist of two ventilators for inlet and exhaust air and a heat exchanger for the heat recovery. A recently developed device, a so called friction ventilator, combines these three elements into a single functional element. The ventilator consists of circular plates which are rotating centrally in between the inlet and the outlet duct of a ventilation system and generate a countercurrent flow in the two ducts. Furthermore, the discs act as a rotating heat exchanger between the two air flows. To increase understanding of the energy transfer from the rotating discs to the flow an experimental investigation on the effect of different rotor geometries was conducted. The study showed an interesting influence of the hub diameter on the characteristic curves with a higher pressure difference for an increase in diameter. The results of the heat recovery measurement however were only mildly affected by the hub geometry. Here the distance between the discs, the rotational speed of the discs and the volumetric flow seemed to have the greatest effect on heat recovery.

A CFD-based Virtual Test-rig for Rotating Heat Exchangers

Rotating heat exchangers are used in steel industry, air conditioning and thermal power plants to pre-heat air used in steam generators or for waste heat recovery. Here we focus on a rotating heat exchanger on a so-called Ljungström arrangement operated in thermal power plants to pre-heat the air fed to the steam generators. In these devices the heat exchange between two fluids is achieved through a rotating matrix that gets in contact alternatively with the two fluid streams and acts as a thermal accumulator. To increase the heat capacity and the overall exchange surface, the rotating matrix is filled by a series of folded metal sheets. In the paper we de-scribe a methodology to account for the effects of the Ljungström in a virtual test-rig implemented in a Computational Fluid Dynamics environment. To this aim, a numerical model based on the work of Molinari and Cantiano was derived and implemented in the OpenFOAM library. RANS numerical results were compared with those of a mono-dimensional tool used by ENEL to design Ljungström heat exchangers and validated against available measurements in a real configuration of a thermal power plant.

Performance enhancement of a desiccant evaporative cooling system using direct/indirect evaporative cooler

In this paper, three novel desiccant evaporative cooling system configurations are proposed, simulated and compared with the conventional system under a wide range of ambient air temperature (30: 40 °C) and humidity ratio (0.01: 0.02 kgv kga−1). In configuration-I, a direct/indirect evaporative cooling is inserted before the rotating heat exchanger. However, it is inserted after the rotating heat exchanger for system configuration-II. In configuration-III, an extra direct/indirect evaporative cooling is added in an opposite manner. Validation results confirm that, simulation and experimental results are in a good agreement with average errors of 2.23 and 3.87% for ambient air temperature and humidity ratio, respectively. Energetic analysis revealed that configuration-I has the highest cooling capacity while configuration-III has the highest thermal COP and air handling COP. Exergetic efficiency of config-III is higher than the conventional system with 54% as average value over the range of ambient air humidity.

Improving heat recovery using retrofitted heat pump in air handling unit with energy wheel

The world is facing a challenge to reduce energy use to meet the environmental goals set for the future. One factor that has a great impact on the energy performance of buildings is the ventilation losses. To handle these losses, heat recovery systems with rotating heat exchanger are often implemented. These systems have been shown to recover about 60–70% of the energy in the exhaust air on an annual basis.After a heat recovery system is installed it is hard to improve the efficiency of the installed recovery system with an acceptable economic payback period. In the present paper one way to improve the energy performance of a building with this type of heat recovery system by the use of a heat pump is investigated by simulations in TrnSys.The heat pump system is arranged so that the evaporator is connected to a heat exchanger mounted in the exhaust airstream after the energy wheel, and the condenser of the heat pump is mounted so that the temperature of return water from the heating coil is increased.The simulations show that there is a possibility to increase the heat recovery rate of the air handling unit in a significant way by retrofitting a heat pump to the system.

Stokes flow heat transfer in an annular, rotating heat exchanger

The heat transfer rate into highly viscous, low thermal-conductivity fluids can be enhanced significantly by chaotic advection in three-dimensional flows dominated by viscous forces. The physical effect of chaotic advection is to render the cross-sectional temperature field uniform, thus increasing both the wall temperature gradient and the heat flux into the fluid. A method of analysis for one such flow—the flow in the eccentric, annular, rotating heat exchanger—and a procedure to determine the best heat transfer conditions, namely the optimal values of the eccentricity ratio and time-periodic rotating protocol, are discussed. It is shown that in continuous flows, such as the one under consideration, there exists an optimum frequency of the rotation protocol for which the heat transfer rate is a maximum.

06/01758 Average temperature of the rotor of high-speed rotating heat exchanger during the period of thermal start-up: Wilk, J. International Journal of Heat and Mass Transfer, 2005, 48, (10), 2079–2089

06/01758 Average temperature of the rotor of high-speed rotating heat exchanger during the period of thermal start-up: Wilk, J. International Journal of Heat and Mass Transfer, 2005, 48, (10), 2079–2089

Average temperature of the rotor of high-speed rotating heat exchanger during the period of thermal start-up

The paper presents results of the calculations of the average temperature variations of model rotor of a high-speed rotating heat exchanger in the course of thermal start up. A model rotor adopted for the calculations has a porous structure formed by radially oriented ducts of a constant circular cross-section. The scope of this paper covers numerical calculations of temperature distribution in the material of the rotor versus its rotational speed, temperature difference of the gases at the inlet, and types of the rotor material. With the use of these results, graphs are elaborated representing the dependence of the rotor’s dimensionless temperature on the Fourier number, with the Biot number used as a parameter.

Perturbation Solution to Mixed Convection in Rotating Horizontal Elliptic Cylinders

A parameter perturbation analysis of laminar free and forced convective heat transfer in rotating horizontal elliptic ducts is investigated. The perturbation parameter used in the solution of the normalized governing equations is the rotational Rayleigh number Raτ , which governs rates of heating and rotation. The results show the influence of rotation and heating on the temperature and axial velocity fields. The effects of Prandtl number, in the range of 0.73 to 4, and eccentricity on the peripheral local Nusselt number are also reported. Results indicate insensitivity of peripheral local Nusselt number to Prandtl number at tube eccentricity e = 0.866, which is an important result to a designer of rotating heat exchanger. The effect of eccentricity on the friction coefficient is also presented. The parameter space for the overall validity of the results presented is 0 < Raτ Rem Pr < ‐ 640. Nomenclature a, b = semimajor and semiminor axes of an ellipse, respectively Cp = specific heat at constant pressure e = eccentricity f (r , θ )= function specifying the temperature distribution in the (r , θ ) plane g = acceleration caused by gravity H = distance between axis of rotation and tube axis i = order of perturbation solution k = thermal conductivity of the fluid Nu(θ )= peripheral local Nusselt number O = tube axis O � = center of the fixed frame of reference P (r , θ )= function specifying the pressure distribution in the (r , θ ) plane

Etude hydrodynamique et thermique d'un échangeur de chaleur à noyau tournant conique

Hydrodynamic and thermal study of a conical rotating heat exchanger. This work reports on a new heat exchanger for various food products (jam, sauces). The fluid flows between two coaxial parallel cones from the vertex to the base. The inner cone rotates and the outer is stationary. A uniform heat flux is delivered through the external wall. The numerical modelling is performed on a bidimensional configuration by the finite-volume method with Phoenics package. The model takes into account the rheological food products characteristics. The principal phenomenon, numerically and experimentally observed, is the appearance of the recirculation that renews the fluid on the heating wall, increases the heat transfer and has a mixing effect.

Analysis of frictionless two-phase flow in tubes under centrifugal acceleration and minimum heating for choked flow

To determine the void fraction in a tube of a rotating heat exchanger, an analytical investigation was undertaken to model frictionless two-phase flow boiling. Steady, one-dimensional separated two-phase conservation equations in differential form, were first applied to a stationary system. The equations were integrated between the inlet and exit of the flow channel to yield three coupled algebraic equations. The algebraic equations were then modified to represent rotating systems. To obtain closure, the velocity ratio, mass quality and void fraction are defined as a function of pressure. A numerical technique was used to solve the equations. Sample results are presented in a graph of mass quality versus void fraction. The graph demonstrates that a minimum heat input must be exceeded to change from a single-phase flow to saturated two-phase flow boiling. Also, the void fraction was found to increase for increasing heat input, decreasing mass flow rate, increasing inlet mass quality and decreasing pressure difference between the inlet and exit.

Friction and wear of plasma-sprayed NiO based coatings

The friction and wear characteristics of plasma-sprayed NiO–CaF2 coatings on Incomel 601 substrates were studied at 760 °C and load of 5.17×104 N/m2. The rubbing surface was a lithia alumina silicate ceramic matrix simulating a rotating heat exchanger for use in automotive gas turbines. Average friction coefficients varied from 0.28 to 0.58 with coating wear rates of 0.2 to 2.1×10−3 cm/100 h. Optimum wear performance was achieved for the prespray composition 92/8 weight % NiO/CaF2.