Browsing by Author "Chang, MY"

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Changes of longitudinal vortex roll structure in a mixed convective air flow through a horizontal plane channel: An experimental study
(1997-01-01)
Chang, MY
;
Yu, CH
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Flow visualization and temperature measurement were conducted to investigate the buoyancy induced longitudinal vortex how structure in a mixed convective air flow through a bottom heated horizontal plane channel for the Reynolds number ranging from 20 to 50 and Rayleigh number From 6000 io 30 000. The results suggested that at low buoyancy-to-inertia ratio a longitudinal roll pair with spanwise symmetry is first induced near the duct sides and more rolls are induced adjacent to the existing rolls as the flow moves downstream. Steady vortex flow prevails after the initial transient. At high buoyancy-to-inertia ratio the splitting and merging of the rolls take place frequently causing the flow to become unsteady and highly asymmetric at high Rayleigh numbers. Several types of the roll splitting processes were identified, namely, the splitting of one roll to three rolls and two rolls to four rolls, formation of two new rolls and two successive roll splittings of the above types. The reverse processes were noted during the roll merging. The data from the measured time histories of the air temperature indicated that the flow oscillation is position dependent with the oscillation amplitude being higher in the downstream core regions. Besides, the oscillation amplitude inside a roll is determined largely by the spanwise or axial position adjustment during the roll splitting and merging processes. Finally, the data for the onset of steady and unsteady vortex flows were empirically correlated. Copyright (C) 1996 Elsevier Science Ltd.
Design and implementation of a real-time lossless dynamic electronic load simulator
(1997)
Chang, MY
;
Lin, JY
;
Jung, SL
;
Tzou, YY
;
交大名義發表
In this paper, the construction of a new power electronics instrument, dynamic electronic load simulator (DELS), is presented. A high-performance full-bridge power amplifier with two-stage output filter and 50 kHz snitching frequency is designed for high bandwidth current loop requirement. The control of the proposed DELS mainly consists of an inner current controller and an outer load-model controller (LMC). Through the simulation and experiments, it is verified that the proposed DELS can replace a heap of RLC power components, exhibit the load behavior approximately and save a large amount of energy during burn-in test.
Design and implementation of an FPGA-based control IC for AC-voltage regulation
(1999-05-01)
Jung, SL
;
Chang, MY
;
Jyang, JY
;
Yeh, LC
;
Tzou, YY
This paper presents a field-programmable gate array (FPGA)-based control integrated circuit (TC) for controlling the pulsewidth modulation (PWM) inverters used in power-conditioning systems for ac-voltage regulation, We also propose a multiple-loop control scheme for this PWM inverter control IC to achieve sinusoidal voltage regulation under large load variations. The control scheme is simple in architecture and thus facilitates realization of the proposed digital controller for the PWM inverter using the FPGA-based circuit design approach. Bit-length effect of the digital PWM inverter controller has also been examined in this paper. The designed PWM inverter control IC has been realized using a single FPGA XC4005 from Xilinx, Inc., which can be used as a coprocessor with a general-purpose microprocessor in application of ac-voltage regulation. Owing to the high-speed nature of FPGA, the sampling frequency of the constructed IC can be raised up to the range that cannot be reached using a conventional digital controller based merely on microcontrollers or a digital signal processor (DSP), Experimental results show the designed PWM inverter control IC using the proposed control scheme can achieve good voltage regulation against large load variations.
Design and implementation of an FPGA-based control IC for the single-phase PWM inverter used in an UPS
(1997)
Jung, SL
;
Chang, MY
;
Jyang, JY
;
Huang, HS
;
Yeh, LC
This paper presents an FPGA-based multiple-loop control scheme for the regulation of the PWM inverters used in UPS. The proposed control scheme incorporates an inner current loop with an outer voltage loop to regulate the output voltage of the PWM inverter, which is expected to be sinusoidal. The corresponding control gains are designed through deadbeat theory so that the inverter can achieve fast dynamic response. An output voltage decoupling mechanism and a load disturbance compensation scheme have been proposed to improve the stiffness of the controlled PWM inverter. The developed digital controller has been realized by a RAM-based FPGA XC4010 to verify its effectiveness. The simulation and experimental results show that the output voltage of the controlled PWM inverter has good transient response and little distortion under rough load conditions.
DSP-based fully digital control of a AC/DC converter with a nonlinear digital current mode control
(1996)
Chang, MY
;
Lin, JY
;
Tzou, YY
;
電控工程研究所
;
Institute of Electrical and Control Engineering
DSP-based multiple-loop control strategy for single-phase inverters used in AC power sources
(1997)
Jung, SL
;
Huang, HS
;
Chang, MY
;
Tzou, YY
;
交大名義發表
This paper presents a fully digital multiple-loop control scheme for the regulation of the PWM inverters used in AC power sources. The proposed digital control scheme incorporates an inner current loop with an outer voltage loop to regulate the output voltage of the PWM inverter. The digital controller is designed through deadbeat theory so that the inverter can achieve fast dynamic response. To enhance the robustness of the PWM inverter concerned, an output voltage decoupling mechanism Is added to improve the performance of the inner current loop. Besides, a load disturbance compensation scheme Is also developed to remedy voltage distortion caused by load variation. Both the frequency domain end time-dom in behavior of the proposed control scheme have been examined in this paper. A PWM inverter control system based on DSP TMS320C14 was constructed to verify the control law. The results show that the output voltage of the controlled PWM inverter has little even under rough load conditions.
Experimental study of aspect ratio effects on longitudinal vortex flow in mixed convection of air in a horizontal rectangular duct
(1998-02-01)
Chang, MY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Combined flow visualization and temperature measurement were carried out to investigate the effects of the aspect ratio on the spatial and temporal characteristics of the longitudinal vortex air how in a bottom heated horizontal rectangular duct. In the experiment, the Reynolds number was varied from 2.5 to 50, the Rayleigh number from 3000 to 20,000 and the aspect ratio from 2 to 12, covering the steady and unsteady longitudinal vortex flows. The results indicated that the spatial vortex flow structure in a low aspect ratio duct is similar to that in a high aspect ratio duct with the central portion of the wider duct around x = A/2 removed, but the unstable vortex how at high Re and Ra can be stabilized when the aspect ratio is reduced. Besides, in a high aspect ratio duct (A greater than or equal to 8) a reduction in the Reynolds number causes the flow to change from steady to unsteady state with more frequent roll splitting and merging. While in a low aspect ratio duct (A less than or equal to 6) at decreasing Re the roll cross-section exhibits cyclic expansion and contraction in time and the longitudinal rolls become snaking and time periodic in the downstream portion of the duct. At a higher buoyancy-to-inertia ratio the snaking motion of the rolls gets more intensive and is initiated at a shorter distance from the duct inlet. When the value of the aspect ratio is an odd number, it was found that there are still an even number of rolls induced in the duct. The transition from steady to time-dependent state at increasing buoyancy-to-inertia ratio is subcritical in a high aspect ratio duct (A greater than or equal to 8). However, in a low aspect ratio duct (A less than or equal to 6) there exists a finite range of the buoyancy-to-inertia ratio for the appearance of the time-periodic snaking vortex flow. Finally, the measured oscillation frequencies of the time periodic snaking vortex how were correlated. (C) 1997 Elsevier Science Ltd.
Flow visualization and numerical simulation of transverse and mixed vortex roll formation in mixed convection of air in a horizontal flat duct
(1997-05-01)
Chang, MY
;
Yu, CH
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Combined experimental flow visualization and three-dimensional unsteady numerical simulation were carried out to investigate the formation of the traveling transverse and mixed vortex roll structures from the initially unidirectional, subcritical flow of air in a bottom heated horizontal flat duct when the Rayleigh number was raised to a supercritical level. In the study the Reynolds number was varied from 2.5 to 20 and the Rayleigh number from 1500 to 8000. Both the experiment and computation, being in qualitative agreement, indicated that in the early stage of the transient a pair of longitudinal rolls are induced in the side wall region; then new longitudinal rolls were induced near the existing ones. Meanwhile, weak transverse rolls were generated in the duct entry. As time proceeds, the transverse rolls grow in size and strength and slowly move downstream to push the longitudinal rolls in front of them out of the duct. At the same time new transverse rolls are generated in the duct entry. Ar a low Reynolds number (Rr less than or equal to 5) the transverse rolls are longer and stronger to expel all the longitudinal rolls out of the test section so that a pure and regular transverse vortex roll structure is formed. While at a higher Reynolds number (Re greater than or equal to 7.5) the transverse rolls are shorter and weaker and a few longitudinal rolls stay, all the time, near the duct sides, resulting in a mixed longitudinal and transverse roll structure. In the numerical simulation the temporal characteristics of the two vortex flow structures at the statistical state are also examined in detail. (C) 1997 Elsevier Science Ltd.
High-performance programmable AC power source with low harmonic distortion using DSP-based repetitive control technique
(1997-07-01)
Tzou, YY
;
Ou, RS
;
Jung, SL
;
Chang, MY
;
交大名義發表
This paper proposes a new control scheme based on a two-layer control structure to improve both the transient and steady-state responses of a closed-loop regulated pulse-width-modulated (PWM) inverter for high-quality sinusoidal ac voltage regulation. The proposed two-layer controller consists of a tracking controller and a repetitive controller. Pole assignment with state feedback has been employed in designing the tracking controller for transient response improvement, and a repetitive control scheme was developed in synthesizing the repetitive controller for steady-state response improvement. Design procedure is given for synthesizing the repetitive controller for PWM inverters to minimize periodic errors induced by rectifier-type nonlinear loads. The proposed control scheme has been realized using a single-chip digital signal processor (DSP) TMS320C14 from Texas Instruments. A 2-kVA PWM inverter has been constructed to verify the proposed control scheme. Total harmonic distortion (THD) below 1.4% for a 60-Hz output voltage under a bridge-rectifier RC load with a current crest factor of 3 has been obtained. Simulation and experimental results show that the DSP-based fully digital-controlled PWM inverter can achieve both good dynamic response and low harmonics distortion.
Structures of moving transverse and mixed rolls in mixed convection of air in a horizontal plane channel
(1997-01-01)
Yu, CH
;
Chang, MY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
A combined experimental and numerical investigation was carried out to explore the structures of the moving transverse and mixed vortex rolls in a low Reynolds number air flow through a bottom heated horizontal plane channel for Re less than or equal to 50. The appearance of the intermittent how patterns characterized by the oscillatory thermal plumes in the upstream core region ahead of the longitudinal rolls at Gr/Re-2 congruent to 35 was experimentally found to be an early indication of the formation of the transverse rolls. At a high Gr/Re-2 there is a high possibility for the plumes to link with their spanwise neighbors to form transverse rolls. The severing of the returning flow, resulting from the blocking of the main flow by the existing transverse rolls, by the thermal plumes produces new transverse rolls. The how visualization revealed that at decreasing Re the vortex flow transformed from the longitudinal to transverse rolls in the sequence of stable longitudinal rolls, unstable longitudinal rolls and a mixture of the longitudinal and transverse rolls. For Re greater than or equal to 7.5 the transverse rolls occupied the core region and the longitudinal rolls exist near the side walls. For Re less than or equal to 5.0 the entire duct is filled with the moving transverse rolls before they become distorted at very high Ra. A correlation equation for the wave speed of the transverse rolls was also proposed. Additionally, a flow regime map delineating different vortex flow patterns was given,The results from the numerical prediction qualitatively supported the experimental observation. Copyright (C) 1996 Elsevier Science.
Structures of moving transverse and mixed rolls in mixed convection of air in a horizontal plane channel
(1997-01-01)
Yu, CH
;
Chang, MY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
A combined experimental and numerical investigation was carried out to explore the structures of the moving transverse and mixed vortex rolls in a low Reynolds number air flow through a bottom heated horizontal plane channel for Re less than or equal to 50. The appearance of the intermittent how patterns characterized by the oscillatory thermal plumes in the upstream core region ahead of the longitudinal rolls at Gr/Re-2 congruent to 35 was experimentally found to be an early indication of the formation of the transverse rolls. At a high Gr/Re-2 there is a high possibility for the plumes to link with their spanwise neighbors to form transverse rolls. The severing of the returning flow, resulting from the blocking of the main flow by the existing transverse rolls, by the thermal plumes produces new transverse rolls. The how visualization revealed that at decreasing Re the vortex flow transformed from the longitudinal to transverse rolls in the sequence of stable longitudinal rolls, unstable longitudinal rolls and a mixture of the longitudinal and transverse rolls. For Re greater than or equal to 7.5 the transverse rolls occupied the core region and the longitudinal rolls exist near the side walls. For Re less than or equal to 5.0 the entire duct is filled with the moving transverse rolls before they become distorted at very high Ra. A correlation equation for the wave speed of the transverse rolls was also proposed. Additionally, a flow regime map delineating different vortex flow patterns was given,The results from the numerical prediction qualitatively supported the experimental observation. Copyright (C) 1996 Elsevier Science.
Unsteady vortex roll structures in a mixed convective air flow through a horizontal plane channel: A numerical study
(1997-02-01)
Yu, CH
;
Chang, MY
;
Huang, CC
;
Lin, TF
;
機械工程學系
A three-dimensional unsteady numerical simulation was conducted to investigate the buoyancy induced vortex how structures in a forced air flow through a bottom heated horizontal plane channel. In particular, a third-order upwind finite difference scheme was chosen to solve the Navier-Stokes and energy equations. In the ranges of the Reynolds number Re from 20 to 50 and Rayleigh number Ra up to 31 000, several roll structures were predicted. In the steady flow at high Re and low Ra after the initial transient the longitudinal rolls possess spanwise symmetry and grow regularly to a fully developed state as it proceeds downstream. While in the time periodic flow at low Re and high Ra the rolls may tilt towards the right side wall and the flow is asymmetric or they may tilt to both sides and are symmetric, depending on the particular case studied. This roll tilting causes the rolls to terminate as they encounter the side walls. Meanwhile, new space is provided in the region near the other side of the duct or near the central vertical plane for the generation of new cells. It is also noted that the rolls are wavy in axial direction. Moreover, the rolls change their sizes during moving downstream and no fully developed state can be discerned. Based on the present data, correlating equations for the onset of convection and Hopf bifurcation were proposed. Copyright (C) 1996 Elsevier Science Ltd.
Unsteady vortex roll structures in a mixed convective air flow through a horizontal plane channel: A numerical study
(1997-02-01)
Yu, CH
;
Chang, MY
;
Huang, CC
;
Lin, TF
;
機械工程學系
A three-dimensional unsteady numerical simulation was conducted to investigate the buoyancy induced vortex how structures in a forced air flow through a bottom heated horizontal plane channel. In particular, a third-order upwind finite difference scheme was chosen to solve the Navier-Stokes and energy equations. In the ranges of the Reynolds number Re from 20 to 50 and Rayleigh number Ra up to 31 000, several roll structures were predicted. In the steady flow at high Re and low Ra after the initial transient the longitudinal rolls possess spanwise symmetry and grow regularly to a fully developed state as it proceeds downstream. While in the time periodic flow at low Re and high Ra the rolls may tilt towards the right side wall and the flow is asymmetric or they may tilt to both sides and are symmetric, depending on the particular case studied. This roll tilting causes the rolls to terminate as they encounter the side walls. Meanwhile, new space is provided in the region near the other side of the duct or near the central vertical plane for the generation of new cells. It is also noted that the rolls are wavy in axial direction. Moreover, the rolls change their sizes during moving downstream and no fully developed state can be discerned. Based on the present data, correlating equations for the onset of convection and Hopf bifurcation were proposed. Copyright (C) 1996 Elsevier Science Ltd.
Vortex flow pattern selection and temporal-spatial structures of transverse and mixed vortex rolls in mixed convection of air in a horizontal flat duct
(1996-11-01)
Chang, MY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Combined Row visualization and temperature measurement were carried out to explore the processes for selecting the buoyancy-induced vortex flow patterns and the temporal-spatial structures of the transverse and mixed longitudinal and transverse vortex rolls in air flow through a bottom-heated horizontal flat duct. The results for 1 less than or equal to Reynolds number Re less than or equal to 50 and 1800 less than or equal to Rayleigh number Ra less than or equal to 30 000 showed that the vortex how patterns were determined from the competition between the buoyancy-induced thermal plume, which tends to generate transverse rolls and the onset of longitudinal vortices in the entry region of the duct. At 70 less than or equal to Ra/Re-2<200 and 6 less than or equal to Re less than or equal to 20, the mixed longitudinal and transverse rolls prevail, while the moving trans-verse rolls dominate for 200 less than or equal to Ra/Re-2<1900 and 1 less than or equal to Re less than or equal to 7.5. Moreover, the measured spanwise distributions of the time-averaged air temperature theta(av) indicated that when theta(av) at the midheight of the duct rises to a value very close to zero, the transverse rolls appear. This simple condition provides a convenient method with which to detect the existence of the transverse rolls. The measured time samples of the instantaneous air temperature showed that in the duct filled with the pure and regular transverse rolls the entire flow oscillates at the same frequency and amplitude. At higher buoyancy, the downstream transverse rolls deform and can even break into irregular cells when Iia is high enough. The corresponding temperature oscillation becomes nonperiodic and its amplitude increases with the downstream distance, In the mixed-roll structure, the axial growth uf the longitudinal rolls tends to bend rbs transverse rolls and causes them to become knotted. Finally, correlation equations for the oscillation frequency of the air temperature: and the wave speed in the transverse and the mixed longitudinal and trails verse vortex flows were provided.
Vortex flow patterns near critical state for onset of convection in air flow through a bottom heated horizontal flat duct
(2001-02-01)
Lir, JT
;
Chang, MY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
In this paper, an experiment is carried out to investigate the buoyancy driven vortex how at slightly subcritical and supercritical Rayleigh numbers in a mixed convective air flow through a bottom heated horizontal flat duct. Particular attention is paid to the flow at a very low Reynolds number for 3.0 less than or equal to Re less than or equal to 5.0. Results from the flow visualization have revealed two new vortex flow patterns in addition to those often seen in literature such as longitudinal rolls (L rolls), moving transverse rolls (T rolls), and mixed longitudinal:transverse rolls (M rolls). The newly observed vortex flow patterns include the stable longitudinal rolls near the duct sides along with nonperiodic traversing transverse waves in the duct core, and the mixed longitudinal and transverse rolls as well as irregular cells. Moreover, steady longitudinal rolls and nonperiodic traversing transverse waves are noted even at subcritical Rayleigh numbers. A flow regime map delineating various vortex flow patterns is given. Temporal characteristics of the flow are also inspected. Furthermore, a correlation is given to estimate the local onset locations of the longitudinal rolls. Meanwhile, the oscillation frequency and convection speeds of the transverse rolls are correlated from the present data. Finally, many complicate processes during the vortex flow formation are noted, such as the generation of the L and T rolls and transverse waves, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the moving and bending of T rolls. (C) 2001 Elsevier Science Ltd. Ail rights reserved.