SYSTEM FOR HEAT TRANSFER OPTIMIZATION USING FLOW IDENTIFICATION SENSORS

Description

BACKGROUND OF THE INVENTIVE FIELD

Heat exchangers play a critical role in efficiently transferring heat from one medium to another. They are used for heating, refrigeration, power generation, and ensuring safe operation of systems. The efficiency of a heat exchanger is measured by the amount of heat transferred between two mediums relevant to the temperature difference between them. An efficient heat exchanger has a significant economic value in reducing the required energy to transfer heat between the two mediums. Moreover, a more efficient heat exchanger can require less space and mass to transfer the same heat as conventional designs.

The efficiency of a heat exchanger can also be characterized by the boiling of the fluid inside the heat exchanger when a two-phase system is utilized. Boiling is an efficient means of transferring heat if the liquid phase maintains contact with the heat exchanger wall, known as nucleate boiling. If boiling increases to a level where a gas layer forms between the heat exchanger wall and the fluid, the heat exchanger efficiency degrades quickly. This is known as the Leidenfrost point, or the onset of film boiling. The optimum heat transfer point for a given temperature difference is the Critical Heat Flux (CHF) point. At the CHF, the fluid inside the heat exchanger boils in a way that effectively transfers heat from the heat exchanger wall to the cooler fluid, without creating a gas layer between the heat exchanger wall and the fluid. In the Nucleate Boiling flow regime, the bubbles leave the heat exchanger wall as soon as they form, maintaining a liquid layer touching the wall. If the critical heat flux is exceeded, the fluid enters a film boiling regime. This can cause a boiling crisis, where the heat transfer decreases rapidly, and the system overheats catastrophically. Once this occurs, recovery can be difficult. To operate a heat exchanger at or near its optimum point, the present invention uses a means of measuring the flow regime of the fluid inside the heat exchanger and controlling the flow to maintain a nucleate boiling regime. In an alternate embodiment, the fluid through the heat exchanger is a hotter fluid where heat is transferred from the fluid to the heat exchanger wall.

In one embodiment, the invention is a thermal management system that uses a flow regime identification instrument (or a flow sensor) to control the heat exchanger operation, so it is maintained at or below the CHF point. It includes integrating the flow regime identification instrument with a controller to control the liquid flow rate and temperature inside the heat exchanger such that the heat flux stays below the CHF point while at the same time being in the nucleate boiling flow regime.

In one embodiment, the present invention provides a universal instrument to optimize the efficiency of a heat exchanger for two-phase systems. The process variable that is optimized is the flow regime staying in the nucleate boiling regime. The set point is a desired flow regime that ensures operation is below the CHF point by a certain factor of safety.

In one embodiment, the present invention uses feedback control schemes to adjust the control variable (i.e., the pump setting for more or less fluid flow) based on the feedback signal from the flow identification sensors. If the flow is operating very close to the CHF point or has passed it, the feedback control increases the coolant fluid flow. Otherwise, the system controls the coolant pump flow for the fluid to be in the nucleate boiling state, compensating for changes in the process conditions. The present invention can be used for heating or cooling optimization, by measuring the colder fluid and controlling it near the critical heat flux.

In one embodiment, the present invention uses multiple control schemes to adjust the control variable (i.e. the pump setting for more or less fluid flow) based on the feedback signal from the flow identification sensors on both sides of the heat exchanger. If the flow is operating very close to the CHF point or has passed it, the feedback control decreases the hot fluid flow, if possible. The decrease in hot fluid flow is also used as a feedback signal to adjust the coolant fluid flow to ensure that both hot and coolant fluids operate close to the optimum CHF points.

The present invention can also be used in multistage cooling where the coolant from one stage is the hot flow to the other. Multistage cooling can be used in applications where a significant decrease in temperature is required, like cryocooling.

The present invention also allows for using volume fraction measurements of a coolant or hot fluid, in addition to flow identification, for further optimization of the system toward the CHF point.

The present invention also allows for fluid velocity and/or mass flow rate measurement for further optimization of cooling.

SUMMARY OF THE GENERAL INVENTIVE CONCEPT

Heat transfer is maximized at the critical heat flux (CHF) point where the heat transfer coefficient over temperature difference between the cooling fluid and the hot surface is at its max. Nucleate boiling occurs at and below the CHF point, maintaining a liquid layer between the coolant and the hot surface. A device that can measure the flow regime can be used to measure if the heat exchange system is operating close to the CHF and adjust the coolant flow rate to make sure the system is operating near the CHF point.

Multidimensional capacitance flow meters were previously developed with the capability to identify flow regimes and measure volume fraction. Those flow sensors can be used to identify the flow regime of the coolant in a heat exchange system toward its optimization.

The present invention provides an innovative design for identifying the flow regime and/or calculating the volume fraction to estimate how far the flow is from the optimum CHF point.

With the ability of measuring the CHF point, the system can be extended to a cascade of subsequent heat exchanger systems where more cooling would be required.

The design of the current invention also preferably includes different control schemes of feedback and cascade feedback. It also includes an embodiment where a hot fluid exists at one side of the heat exchanger and a cooling cold fluid at the other side. The maximum heat exchange rate in this case is achieved when both the hot and the cold fluids are operated at their CHF points.

The design of the present invention includes identifying the flow regime by comparing measurements from plate combinations in a capacitance sensor, analysis of time-series capacitance measurements, or by capacitance tomography imaging of the flow.

In one embodiment of the invention, the invention is comprised of:

• • a heat transfer optimization system comprised of a first heat exchanger;
    a flow sensor for obtaining flow regime information or data for fluid flowing through the first heat exchanger, a flow regulating device for controlling the rate of flow of the fluid flowing through the first heat exchanger; a control system that uses flow regime information or data to send control signals to the flow regulating device to control the rate of flow of the fluid flowing through the first heat exchanger to control the rate of heat transfer. The system is preferably programmed to keep the fluid flowing through the first heat exchanger at or close to a Critical Heat Flux point. The system is preferably configured to control the rate of flow of the fluid flowing through the first heat exchanger to maintain the fluid flowing through the first heat exchanger at or close to a nucleate boiling regime. In one embodiment, the system is further comprised of a fluid pump or valve for controlling the temperature of a fluid outside the first heat exchanger to optimize heat transfer between the fluid outside the first heat exchanger and the fluid flowing through the first heat exchanger. The foregoing and other features and advantages of the present invention will be apparent from the following more detailed description of the particular embodiments, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example nucleate flow regime development as a cold liquid is passed through a pipe of hot surface.

FIG. 2 illustrates the CHP point where the heat flux over temperature difference is at its max.

FIG. 3 illustrates one embodiment of a system for controlling heat transfer optimization using flow identification sensor(s).

FIG. 4 illustrates another embodiment of a system for controlling heat transfer optimization using flow identification sensor(s).

FIG. 5 illustrates another embodiment of the present invention using the operation of a cascade system having a plurality of heat exchangers, flow meters/sensors and control loops.

FIG. 6 illustrates one embodiment of the control system signal path.

FIG. 7 illustrates an alternate embodiment of the control system signal path.

FIG. 8 illustrates one embodiment of the invention where the heat exchanger is used as part of the capacitance sensor's sensing plates.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

The following detailed description of the example embodiments refers to the accompanying figures that form a part thereof. The detailed description provides explanations by way of exemplary embodiments. It is to be understood that other embodiments may be used having mechanical and electrical changes that incorporate the scope of the present invention without departing from the spirit of the invention.

Multi-dimensional capacitance sensing is a technology that senses measured capacitances between sensor plates to collect information about each phase in a multi-phase flow system. It has provided insights into multi-phase flow phenomena in many industrial processes often in a combination of gas, liquid, and solid states, including cryogenic gas-liquid flows, pneumatic conveying, gravity drop flows, oil pipe lines, geothermal fluid flow, fluidized beds, bubble columns and many other chemical and biochemical processes. It may also be used for measuring flows in biological processes. See U.S. Pat. Nos. 8,614,707, 10,269,171, 10,806,366 and 10,705,043, and U.S. Patent Publication 2018/0325414 incorporated by reference herein. In the preferred embodiment of the invention, the present invention utilizes a multi-dimensional Data Acquisition System (DAS) technology for the electronics, and ECVT and/or adaptive ECVT for plate formation. The ECVT sensor distributes the electric field in 3D, which could be used in the present invention to place plates in a way that provide accurate volume fraction.

Multi-dimensional DAS technology was previously developed to measure capacitance between plates of a capacitance sensor based on excitation of the sender electrodes with sinusoidal signals of different frequencies. The measured signal amplitude and/or phase is then used to calculate the volume fraction of flow phases.

Multi-dimensional flow meters or flow sensors have also been developed for calculating the volumetric flow rate of a multi-phase flow using the same sensor used for measuring the volume fraction. See, e.g., U.S. patent application Ser. No. 18/590,693 incorporated by reference herein. The velocity is measured by cross-corelating measurements of the sensor in multiple layers (e.g., layers of plates). The distance between layers in the sensor determines the velocity resolution. The sensor also preferably provides correlation signals from plates in the same layer for cross-sectional flow velocities.

The interactive design of the multi-dimensional flow meter or flow sensor also preferably includes a temperature sensing mechanism for applications that involve wide variations in flow temperature. For example, a temperature sensor is established to adjust flow components densities for accurate estimation of mass flow rates.

The interactive design of the multi-dimensional flow meter or flow sensor also includes identifying the flow regime by comparing measurements from different plate combinations. Flow regime relates to distribution styles of the flow. More specifically, it includes bubbly flow, stratified, stratified-wavy, plug, slug, annular, intermittent, mist flow, and turbulent flow.

In one embodiment of the multi-dimensional flow meter or flow sensor, the flow meter or flow sensor is comprised of: a capacitance sensor having a plurality of plates, a data acquisition circuit in communication with the capacitance sensor, wherein the data acquisition circuit receives input data from the capacitance sensor including current output from the capacitance sensor, one or more processors operationally connected to the data acquisition circuit, wherein the one or more processors receive a signal from the data acquisition circuit and wherein the one more processors are configured to extract amplitude or phase data from the signal, a thermal sensor operationally connected to the data acquisition circuit, a non-transitory computer-readable medium including one or more sequences of instructions that, when executed by the one or more processors, cause the one or more processors to obtain a mass flow rate of a single-phase flow based on information received from the thermal sensor.

The present invention relates to a heat transfer optimization system composed of a flow sensor (e.g., using a multi-dimensional flow meter), a control system that uses the flow characteristics and/or flow regime data obtained by the flow sensor to adjust flow rate, and a flow regulating device (e.g. pump or valve) or devices that respond(s) to control system signals in order to control the temperature in the system to operate the heat exchanger at or near its optimum point. This can be done by controlling the rate of the flow of the fluid input into the heat exchanger and/or controlling the temperature of the fluid (liquid or gas) around the heat exchanger (e.g., by controlling a heated fluid pump). The heat transfer system is programmed to keep the system fluid at or close to the Critical Heat Flux (CHP) point.

FIG. 1 illustrates an example nucleate flow regime development as a cold liquid is passed through a pipe of hot surface. FIG. 1 depicts fluid boiling of a coolant as it passes through a hot surface. FIG. 2 shows the CHP point where the heat flux over temperature difference is at its max. The optimum operating state is at the CHF. The figure shows system operation near critical heat flux. The most efficient heat transfer point is at the CHF point as depicted in FIG. 2. At this point, the ratio of heat flux over temperature change is maximized, yielding efficient heat transfer. In the preferred embodiment, the optimum state that the system is controlled at is set with a certain factor of safety below the CHF.

FIG. 3 illustrates one embodiment of a system 10 for controlling heat transfer optimization using flow identification sensor(s) or flow sensor 12. This embodiment is a management system where a flow sensor is used to measure when flow boiling happens and to what extent by obtaining/measuring the flow regime information or data, volume fraction and/or other signal characteristics as well (collectively referred to as “flow identification information or data”). The flow identification information is used to control the flow regulating device 14 (e.g., pump or valve) to maintain the heat flux at the CHF point or close to it. The first heat exchanger 16 is preferably made of metal or ceramic, but it can be made of any conductive material.

In the embodiment shown in FIG. 3, a second heat exchanger or condensing radiator 18 is operationally located between the first heat exchanger and the flow regulating device for cooling the fluid flowing through the heat exchanger. The receiver tank 20 is a fluid reservoir where the pump pulls liquid from to circulate again through the loop.

In this embodiment, the controller is a PID, microcontroller, or other computation device embedded in the system as part of the flow sensor. The controller can also be separate from the flow sensor. The action out of the controller is to open or close valves, or to increase or decrease fluid flow rate from pump. The flow sensor, in one embodiment, can be a multidimensional capacitance sensor which involves activating the capacitance sender plate sensor(s) or plate(s) with sinusoidal excitation that can be of a single or multiple frequencies and detecting a signal amplitude, phase, or both from receiving sensor(s) or plate(s). In its simplest form, the sender plate is excited with a single frequency excitation signal and the receiver plate records the receiver signal amplitude or phase.

FIG. 4 illustrates another embodiment of a system for controlling heat transfer optimization using flow identification sensor(s). This embodiment is for the operation of a heat exchanger with two fluids, one coolant and another hot fluid (e.g., gas or liquid outside of the heat exchanger) to be cooled. The critical heat flux can be controlled by adjusting the flow rate of the heated fluid and/or coolant fluid independently. In the preferred embodiment, a multi-dimensional flow sensor, using capacitance sensors as described above, is placed around the heat exchanger in order to detect flow regime in order to control the system for more efficient heat transfer (e.g., if the flow sensor detects too much gas/bubbles in the flow, this may indicate that the optimal CHF conditions are exceeded and thus the system should adjust the flow rate of the heated fluid and/or coolant fluid independently in order to get the system back to optimal conditions). The embodiment of FIG. 4 has a second flow regulating device 22 (e.g., pump or valve) for regulating the flow of hot liquid to maintain the heat flux at the CHF point or close to it based on the detected flow identification information. The second flow regulating device is configured to control the temperature of a fluid outside the heat exchanger to optimize heat transfer between the fluid outside the heat exchanger and the fluid flowing through the heat exchanger.

FIG. 5 illustrates another embodiment of the present invention using the operation of a cascade system having a plurality of heat exchangers, flow meters/sensors and control loops. The system optimizes each of the hot and cold fluid flow rates to operate near the CHF points in each of the heat exchangers, so the system overall is operating efficiently. A flow regime identification sensor (e.g., a multi-dimensional flow meter) measures flow regime data at each stage. A control system uses the flow regime data from each stage to adjust fluid flow rate at each stage. This heat transfer system is programmed to keep the system fluid at or close to the Critical Heat Flux point at each stage of the cascade system. Control signals could be from one central controller or controlled independently. The cascade embodiment works for example, as if each stage removed 20° of heat, for a total cooling power of 60° in a 3 stage system, where each stage would be independently optimized. And each stage could consist of a fluid with a different boiling point.

FIG. 6 illustrates one embodiment of the control system signal path. The flow regime identification from the heat exchanger is used to calculate the degree of flow boiling and how far away it is from the CHF point. This value is then compared to a set point representing the CFH and the outcome of the comparison is used to increase or decrease the flow coolant through a pump.

FIG. 7 illustrates an alternate embodiment of the control system signal path. The flow regime identification from the heat exchanger is used to calculate the temperature/degree of flow boiling and how far away it is from the CHF point. This value is then compared to a set point representing the CFH and the outcome of the comparison is used to increase or decrease the flow rate of the hot fluid if possible, or the coolant flow through a pump.

FIG. 8 illustrates one embodiment of the invention where the heat exchanger 24 is used as part of the capacitance sensor's sensing plates. The figure represents the cooling of a surface (e.g., a cable carrying current) being surrounded by flowing coolant fluid 26. The coolant fluid reduces the temperature of the hotter surface 28 (e.g., cable). The hot surface is also used as a capacitance sensing plate. Plates on the outer surface of the embodiment are receiver plates 30. Signals from the receiver plates are used to measure capacitance between the hot surface being cooled and the receiver plates for flow regime measurement/identification toward optimization of heat transfer close to the CHF point. In other words, the flow regime data and/or volume fraction data obtained by the capacitance sensor (of the flow sensor or meter) is used to send control signals to a flow regulation device (e.g., pump or valve) that are used to control the rate of flow of the fluid flowing around the heat exchanger to control the rate of heat transfer.

Although the aforementioned describes embodiments of the invention, the invention is not so restricted. It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments of the present invention without departing from the scope or spirit of the invention. Accordingly, these other types of sensors, controllers, flow regulating devices and heat exchangers are fully within the scope of the claimed invention. Therefore, it should be understood that the apparatuses and methods described herein are illustrative only and are not limiting upon the scope of the invention, which is indicated by the following claims.