Generation and Manipulation of Higher Order Fractional and Integer Bessel Gaussian Beams

Optical orbital angular momentum (OAM) describes orbiting photons, swirling local wave vectors, or spiraling phase distribution depending on what theory we use to explain light. If we consider light as a propagating electromagnetic wave, then light has the freedoms of frequency, magnitude, phase, and polarization. For a monochromatic light, expanding the later three freedoms spatiotemporally, numerous optical modes are solved from Maxwell’s equations and boundary conditions. OAM mode study starts from integer charge because it is in the integer form of the fundamental phase singularity structure. Fractional OAM mode is the Fourier series of integer OAM modes. The average OAM does not conserve along with propagation for the traditional fractional OAM modes. We propose a new asymmetric fractional Bessel Gaussian mode providing the average OAM conserving along with the propagation. To better understand the fractional OAM mode or integer OAM mode combination, we study the novel concentric vortex optics. The analytical propagation expression of the concentric vortex beam is derived and analyzed. The concentric vortex beam is essentially the OAM spectrum, with only two integer OAM components. The spectrum coefficiencies are real numbers and approximately power equalized in general cases. The concentric vortex beam is the coherent combination of incomplete Kummer beams. As the inner aperture tuning large, the beam evolves into the Kummer beam with the inner charge number. The aperture decreases, the outer charges Kummer beam dominates. The proposed asymmetric fractional Bessel Gaussian beam’s Fourier transform is azimuthal Gaussian perfect vortex. We use log-polar coordinate mapping diffractive optics to transform the elliptical Gaussian beam into the desired azimuthal Gaussian perfect vortex beam. The generated asymmetric fractional Bessel Gaussian beam is systematically compared with Kotlyar’s asymmetric Bessel Gaussian beam. It’s found that the proposed beam has a narrower OAM spectrum, preserving average fractional OAM. Furthermore, the log-polar transform’s inherent output lateral shifting problem is addressed for the first time to our knowledge. An improved log-polar design is proposed, and we use five critical metrics to show the new log-polar generated asymmetric Bessel Gaussian beam’s quality is much improved. The manipulation of the high order asymmetric fractional Bessel Gaussian beam is critical to applications scaling from communication, sensing, filamentation, to micromanipulation. We propose and demonstrate acousto-optical deflector (AOD) HOBBIT (Higher Order Bessel Beams Integrated in Time) system. The system can continuously tune the OAM modes on the order of 400 kHz. This speed beats the fastest spatial light modulator (SLM), and even better, the proposed system could work for high power applications

Two-phase Refrigerant Flow in the Inlet Header of Brazed Plate Heat Exchangers: Visualization and Its Effect on Distribution

When used as direct expansion (DX) evaporators, brazed plate heat exchangers (BPHEs) suffer from the maldistribution of the two-phase refrigerant flow among parallel plate channels. One of the critical mechanisms of two-phase maldistribution is the phase separation in the inlet header. This paper presents an experimental investigation of the two-phase refrigerant (R134a) flow in the inlet header of a brazed plate evaporator and its effects on the distribution. Visualization of two-phase flow is accomplished through a 3D-printed transparent window at the inlet header of the heat exchanger. The observed flow regime is periodic and three stages are identified in a cycle. The influence of the operating conditions on the two-phase flow regime is also explored. The two-phase flow distribution in the BHPE is quantified by an IR thermography-based method. The quantification results demonstrate that the two-phase flow regime in the inlet header significantly affects the distribution

An Infrared Thermography Based Quantification Method of Two-Phase Refrigerant Distribution in Brazed Plate Heat Exchangers

When used as direct expansion (DX) evaporators, brazed plate heat exchangers (BPHEs) suffer from the maldistribution of the two-phase refrigerant among parallel plate channels. It is essential to accurately quantify the two-phase refrigerant distribution in BPHEs to evaluate the effect of the maldistribution on the heat exchangers and system performance. Considering the complex internal geometry of BPHEs and the potential influence of experimental measurements on the original flow field, the non-intrusive quantification of the two-phase flow distribution in BPHEs is preferred. This paper presents a method to quantify the two-phase refrigerant distribution in BPHEs from infrared images. Quantification is achieved by identifying the boundary between the two-phase and superheated region on the sidewall of BPHEs based on the infrared images and matching the identified boundary in a BPHE evaporator model to estimate the two-phase distribution. The proposed quantification method is validated by the experimental data. The results show that this non-intrusive method can effectively quantify the two-phase refrigerant distribution in the BPHE

Single Phase Pressure Drop and Flow Distribution in Brazed Plate Heat Exchangers

Brazed plate heat exchangers (BPHE) have been widely used in the heating, ventilating, air conditioning, and refrigeration industry, but refrigerant distribution among parallel plate channels is still one of the main issues. Maldistribution of refrigerant among different plate channels is greatly affected by the pressure changes along the inlet and outlet header, and it would generally decrease the performance of BPHE by causing higher pressure drop and poor utilization of heat transfer area. In this paper, the experimental and simulation methods are used to study the single-phase pressure drop and flow distribution in a U-type BPHE. Experiments are conducted to measure the pressure changes along the inlet and outlet header, as well as the pressure difference through each plate channel. And the CFD tool is used to simulate the flow details in the inlet and out header of the BPHE

Transient Distribution of Refrigerant and Oil in A Residential Heat Pump Water Heater System

This paper presents an experimental investigation of transient refrigerant and oil migration in a residential heat pump water heater (HPWH) system. In the experiments, R134a is paired with POE 22 oil. The Quick Closing Valve Technique (QCVT) is employed to localize the refrigerant and oil into each section of the system. The Remove and Weigh Technique (RWT) is then applied to measure the trapped refrigerant mass in the sections, with an uncertainty about 0.17% of the total refrigerant charge. The retained oil mass in each section, except for the compressor, is determined by the Mix and Sample Technique (MST), of which the uncertainty is about 0.15% of the total oil charge. Five experiments are conducted to cover a full heating-up of five hours. The experimental data shows that the most of refrigerant is in the heat exchangers. The inventory of the refrigerant generally decreases in the evaporator and increases in the condenser during the heating-up. The measurements also indicate that most of the oil stays in the compressor. The retention of the oil generally increases in the evaporator and it first decreases then increases in the condenser

An Experimentally Validated Model of Single-Phase Flow Distribution in Brazed Plate Heat Exchangers

This paper presents a mathematical model to predict single-phase flow distribution in brazed plate heat exchangers (BPHEs). In this model, the flow distribution among parallel channels is predicted by imposing the condition that each flow path in the heat exchanger has an identical total pressure drop. The model has been validated by the experiments, in which the pressure drop of gas flow in the headers, as well as across the channels is measured. This model is then used to explore the effect of maldistribution on the thermal performance of the heat exchangers. The experimental and modeling results show that, for a U-type BPHE, the channel mass flow rate generally decreases along the flow direction in the inlet header. The single-phase maldistribution is highly depending on the relative magnitude of the pressure change along the headers and the pressure drop through the plate channels. For given heat transfer area, a heat exchanger with a longer plate would benefit from less non-uniformity in mass and higher heat transfer coefficient, but suffer from higher overall pressure drop

Transient distribution of refrigerant and oil in a residential heat pump water heater system

In most vapor-compression refrigeration systems, oil is added into the compressor for lubrication. However, it is inevitable that a portion of oil escapes from the compressor and circulates throughout the system due to the mutual solubility between the refrigerant and oil. The presence of circulating oil would affect the characteristics of heat transfer, pressure drop and mass retention in system. In addition, a large amount of retention of oil outside of the compressor might cause insufficient lubrication of the compressor, and eventually lead to compressor failure. The objective of this thesis is to experimentally and numerically investigate the transient refrigerant and oil distribution in a residential heat pump water heater (HPWH) system. In the experiments, R134a is used to pair with POE 22 oil as the working fluid. Quick Closing Valve Technique (QCVT) is employed to localize refrigerant and oil into each component of the system. Remove and Weigh Technique (RWT) is then used to measure the refrigerant mass, with an uncertainty about 0.17% of total refrigerant charge. The retained oil mass in each component, except for the compressor, is determined by Mix and Sample Technique (MST), of which the uncertainty is about 0.15% of total oil charge. Five experiments are conducted to cover a full heating process of five hours. The experimental data shows the retention of refrigerant is mainly determined by the internal volume and refrigerant density in the component. The retention of oil is found depending on the velocity of liquid refrigerant-oil mixture. A linked EES-CFD system model has been developed to simulate the transient system performance of the HPWH unit. Experimental data is used to validate this model. A retention model has also been established to analyze the local refrigerant and oil distribution in the heat exchangers