12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics VISUALIZATION OF THE FLOW DISTRIBUTION INSIDE THE PISTON DISPLACEMENT OF A GAMMA-TYPE STIRLING ENGINE J. Luis Luviano-Ortiz, C. Ulises Gonzalez-Valle, Guillermo Hernandez-Cruz, Eduardo Ramos, Abel Hernandez-Guerrero, Sindy Tarazona-Cardenas* Department of Mechanical Engineering, University of Guanajuato, Salamanca, Guanajuato, Mexico E-mail: [email protected] ABSTRACT thermal auto oscillations systems or swash plate drive [1]. A The dynamics of the confined flow inside the piston different dynamic behavior can be achieved by the displacement of a Gamma-type Stirling engine operating as a implementation of each system. heat pump is experimentally analyzed. The diameter of the piston is 30 mm; 2 mm smaller than the internal diameter of the There are several types of Stirling engines. The Gamma- cold cylinder. As the piston performs its reciprocating motion, type is one of the most common and widely studied Stirling the gas leaks around the gap between the piston and the engines. The Gamma-type configuration has two cylinders; cylinder wall, generating a continuous pulsating motion. In the displacer and power piston are used (similar to Beta-type first part of the cycle air is introduced and compressed by the engines). This configuration allows a complete separation piston motion, and then the second part of the cycle starts by between the displacer, where the heat exchange occurs, and the expanding the air and ejecting it from the assembly chamber. expansion chamber where the power piston is located [2]. The Experimental observations were carried out at frequencies in device analyzed in this study is a commercial Stirling Engine G the range 100 to 300 rpm. The flow was visualized using a U10050 manufactured by 3B SCIENTIFIC® PHYSICS [3,4]. vertical laser beam plane oriented in the same axial direction of the piston’s motion. The particles used as trackers are water NOMENCLATURE drops condensed on carbon dioxide microcrystals. Images were taken with a high speed video camera with a frame rate of 1000 X [m] Displacer displacement fps. PIV techniques were implemented to identify the flow R [m] Crank length L [m] Rod length main structures. For analysis purposes, fixed phase averages of U [m s-1] Displacer linear velocity the velocity fields are required, due to the turbulent regime A [m s-2] Displacer acceleration observed in this phenomena and its oscillatory nature. Based on T [s] Time experimental measurements it can be demonstrated that the Special characters average flow involved is not axisymmetric, although very ω [rad s-1] Angular velocity interestingly, specific inlet and exit regions of the piston- θ [rad] Crank angle cylinder gap were identified. A Particle Image Velocimetry (PIV) technique is INTRODUCTION implemented to visualize the fluid response inside the displacer. Stirling engines are thermal devices that are able to This type of procedure allows to determine important transform heat into mechanical work in a very efficient cycle. parameters and to picture some important phenomena effected This can be achieved by forming a Stirling thermodynamic inside the studied area. Several PIV techniques have been cycle between a heat source and a heat sink. As a thermal carried out throughout numerous experimental analyses. A PIV machine its efficiency is restricted by the Carnot efficiency, technique involves little particles that are tracked over time both are theoretically equal but in practice this efficiency value along the structure that needs to be analyzed, thus the local cannot be achieved. velocity of the fluid can be determined [5]. Quite a few particles can be used as trackers. Table 1 presents the main The ideal Stirling cycle includes two isothermal seeding materials implemented for liquid and gases. In this processes and two constant volume processes. Theoretically the work a novel PIV technique is implemented; this PIV technique absorption and rejection of heat are carried out at constant seeds dry ice particles as trackers. A few works implementing temperature. Along the thermodynamic cycle the volume inside this technique are reported in the technical literature [6-8]. An the chamber changes due the pistons motion and heat is important factor to take care of is the clarity of the obtained transferred simultaneously. This kind of devices can be used as images; this technique allows very clear shoots. Regularly air an engine or as a heat pump. For the heat pump operation an PIV is done by seeding particles such glycerin due its refraction external driven force must be implemented. Several motion index [9]. The refraction index of this material is nearly equal mechanisms can be used to drive this device. The motion to the glass, allowing the light to pass through the visualized mechanisms include rhombic drive, cam-drive, mass/spring area without suffering any diffraction. Although this effect the 999 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics glycerin is not the adequate tracker particle due the soiling inside the chamber. This set up is shown in Figure 1. The caused inside the chamber of the pistons. So, due to its volume of the internal piston chamber is approximately 35 ml. cleanliness the dry ice is selected as the tracker particle in this study. As mentioned before, the Stirling engine is operated as a heat pump by coupling the Stirling device to a DC motor. The Table 1. - Main seeding materials for liquid and gases visualization [5]. DC motor velocity is controlled by an interface between the Seeding materials for liquid flows. device, a power source (Agilent E3632A), and a CPU; in order Type Material Mean diameter [μm] Solid Polystyrene 10-100 to achieve the correct configuration it was necessary to build an Aluminum flakes 2-7 encoder. The encoder allows the communication of the angular Hollow glass spheres 10-100 velocity value to the CPU through a data acquisition card NI Granules for synthetic 10-500 USB-6008. While data was acquired through the DAQ, the coatings Liquid Different oils 50-500 CPU indicated the power source the adequate amount of Gaseous Oxygen bubbles 50-1000 voltage provided to the DC motor for maintaining a proper Seeding materials for gas flows. mean angular velocity (ω). These processes were taken through Type Material Mean diameter [μm] a LabView® interface. Solid Polystyrene 0.5-10 Alumina Al O 0.2-5 2 3 The displacement of the MAIN piston was measured by an Titania TiO2 0.1-5 Glass micro-spheres 0.2-3 accelerometer (Arduino MPU6050) connected to a data Glass micro-balloons 30-100 acquisition card NI USB-8452. For the pressure variations Granules for synthetic 10-50 measurements an Omega Engineering PX-105 transducer was coatings Dioctylphathalate 1-10 used. For temperature measurements two type-T thermocouples Smoke <1 were implemented. These three measuring devices were Liquid Different oils 0.5-1.5 installed through a data acquisition card NI PCI-6040E Di-ethyl-hexyl-sebacate 0.5-1.5 connected to the CPU. The sensing interval used in the (DEHS) Helium-filled soap bubbles 1000-3000 accelerometer, the pressure transducer, and the thermocouples is Δt=0.001 s, assuring enough experimental information since In this work the complete instrumentation of a Gamma- the mean angular velocity of the Stirling engine was 200 rpm. type Stirling engine was carried out. Temperature, pressure and acceleration measurements were taken and reported. A simple Once the experimental data from the accelerometer was kinetic analysis was developed and compared with the obtained, this data is integrated, once for velocity and twice for experimental data reported presenting a good accuracy with the displacement of the power piston. Following the gamma experimental results. Flow visualization of the displacer is configuration of the Stirling engine it is well known that the presented and discussed for several positions of the piston. displacer is 90º out of phase with respect to the power piston, due to this fact information for the displacer can be obtained RESEARCH DESCRIPTION from the power piston measurements. The main purpose of this work lies in the instrumentation of a Gamma-type Stirling engine as well as the implementation of KINETCS OF A GAMMA-TYPE STIRLING ENGINE a Particle Image Velocimetry technique used when the device is As mentioned before the Stirling engine was coupled to a operated as a heat pump. A commercial Stirling engine model DC motor with an elastic band allowing the angular velocity of U10050 [3] is used and shown in the Figure 1. the engine not to be constant. This is induced due the counterweights connected to both pistons, these counterweights avoid that the engine stops when the pistons reach their critical positions. Figure 2 shows the counterweights used on this device. It is important to mention that the displacer has two counterweights while the power piston only has one counterweight. Figure 1 Stirling Engine U10050 [3]. This device has two connections mounted on the piston Figure 2 Counterweights attachment configuration in the stroke for temperature measurements and a connection on the Stirling engine used in this analysis. opposite side of the power piston in order to measure pressure 1000 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics Experimental data from the accelerometer is used; this data experimental and the theoretical data. It can be inferred that the is presented in Figure 3; the data obtained by an integration small changes presented in the angular velocity and the procedure is also shown in Figure 3. This data is compared with acceleration due to the counterweights have not notorious effect the kinetic analysis of the Stirling engine. Equations (1-3) on the displacement and linear velocity. present the motion expressions for a Stirling engine.