IEEE Transactions on Components, Packaging and Manufacturing Technology
This study quantitatively compares the finite element and lumped modeling techniques to analyze p... more This study quantitatively compares the finite element and lumped modeling techniques to analyze phase-change and multiphase flow in a microchannel evaporator. We compare the one-zone and two-zone models in the lumped modeling approach by considering both pressure drop and phase change in the evaporator. The study investigates the deviation in results from the lumped model relative to the finite element model for various evaporator heat loads. For the one-zone model, the relative errors in predicting the average density, pressure, temperature, and vapor quality increase with the evaporator heat load. This increase is mainly due to the nonlinear variation in the heat transfer coefficient and pressure drop, which are calculated with higher inaccuracies at larger evaporator heat loads. On the other hand, the liquid-phase region and the exit vapor quality are predicted more accurately. The overall errors using one-zone and two-zone lumped models were less than 30% when the exit qualities do not exceed 0.5. Since strategies involving flow boiling in evaporators typically avoid high exit vapor qualities to circumvent dry-out and critical heat flux, the use of lumped models under these conditions is favorable. This approach also avoids the computational costs associated with finite element models while achieving sufficient accuracy to predict the evaporator’s performance.
— This paper presents a modeling and control method for the thermohygrometric condition (temperat... more — This paper presents a modeling and control method for the thermohygrometric condition (temperature and humidity) in a multizone building. The interconnection between the zones is captured through an undirected graph. Employing an electrical circuit analogy, rooms represent capacitances, and walls and doors/windows are resistances. This model characterizes both mass and heat transfer between the zones and their coupling within each zone, extending the temperature-only resistance– capacitance models commonly used in the building control literature. By using physics-based computational fluid dynamics (CFD) simulation, we verify that this lumped-parameter model is a reasonable approximation of the physical system. The control objective is to drive the temperature and humidity of each zone into the comfort region in the psychrometric chart, using the mass-flow rate of the supplied air into the zone as the control input. In contrast to thermal-only building control, the challenge of this problem is to use a single control variable, mass-flow rate, to regulate both temperature and humidity. Our approach is to first design a feedforward control based on the desired steady-state condition within the comfort zone. We then draw on our previous work on passivity-based building temperature control to show that the thermohygrometric model around the steady state is strictly passive, from the mass-flow rate to a synthetic output combining temperature and humidity. This allows the use of any passive feedback controller combined with the feedforward to achieve robust stabilization about the desired operating point. The feedforward may be further adaptively updated, resulting in an integral-control term in the controller. Finally, to reduce the energy usage, we only apply the controller outside of the comfort zone and turn OFF the controller within the comfort zone. To illustrate the effectiveness of the proposed control strategy, simulation results using both the lumped and CFD models are presented for an existing physical six-room testbed.
Predicting and controlling the flow regime transition of multiphase fluids in microchannels is es... more Predicting and controlling the flow regime transition of multiphase fluids in microchannels is essential for various energy applications, such as flow boiling, de-emulsification and oil recovery processes. This in turn requires a better understanding of multiphase flow behaviors in microchannels with various channel surface wettability, fluid interfacial tension and flow rates. In this paper, experiments and Lattice Boltzmann method (LBM) simulations are carried out to study complicated multiphase flow at micro or meso scales. With the Shan-Chen multiphase LBM model, the flow pattern transitions of adiabatic two phase flow in a microchannel were investigated. The effects of surface wettability and liquid/gas velocity ratio on the flow regime transition were further studied. A series of two-phase flow experiments were conducted on a PDMS microfluidic device under different gas/oil velocity ratios. Under various surface wettability conditions, our simulation results agree well with the flow visualization experiments equipped with a high speed camera (HSC). Our finding shows that the cross-section meniscus curve width, corresponding to the shadow in the HSC photo, increases with decreasing contact angle, which was confirmed by the simulated liquid/gas distribution. Besides the influence of surface wettability, the role of gas/liquid velocity ratio on two-phase flow regime transition was discussed in detail. The proposed approach paves the way to probe complicated physics of multiphase flows in microporous media.
— This paper presents a modeling and control method for the thermohygrometric condition (temperat... more — This paper presents a modeling and control method for the thermohygrometric condition (temperature and humidity) in a multizone building. The interconnection between the zones is captured through an undirected graph. Employing an electrical circuit analogy, rooms represent capacitances, and walls and doors/windows are resistances. This model characterizes both mass and heat transfer between the zones and their coupling within each zone, extending the temperature-only resistance– capacitance models commonly used in the building control literature. By using physics-based computational fluid dynamics (CFD) simulation, we verify that this lumped-parameter model is a reasonable approximation of the physical system. The control objective is to drive the temperature and humidity of each zone into the comfort region in the psychrometric chart, using the mass-flow rate of the supplied air into the zone as the control input. In contrast to thermal-only building control, the challenge of this problem is to use a single control variable, mass-flow rate, to regulate both temperature and humidity. Our approach is to first design a feedforward control based on the desired steady-state condition within the comfort zone. We then draw on our previous work on passivity-based building temperature control to show that the thermohygrometric model around the steady state is strictly passive, from the mass-flow rate to a synthetic output combining temperature and humidity. This allows the use of any passive feedback controller combined with the feedforward to achieve robust stabilization about the desired operating point. The feedforward may be further adaptively updated, resulting in an integral-control term in the controller. Finally, to reduce the energy usage, we only apply the controller outside of the comfort zone and turn OFF the controller within the comfort zone. To illustrate the effectiveness of the proposed control strategy, simulation results using both the lumped and CFD models are presented for an existing physical six-room testbed.
— This paper presents a modeling and control strategy for comfort zone set-based control of tempe... more — This paper presents a modeling and control strategy for comfort zone set-based control of temperature and humidity in buildings. We first propose a coupled model for humidity and temperature dynamics based on lumped parameter analysis. The interconnection of rooms/zones is captured through an undirected graph, with rooms represented as capacitances and walls and doors/windows as resistances. Unlike traditional RC-models, however, this model captures both mass and heat transfer between zones as well as the bilinearity in the input mass flow-rate. Key parameters are identified by the model, such as mass (and thermal) conductance between zones as well as mass (and thermal) capacitance and this model structure is then validated using physics-based Computational Fluid Dynamics (CFD) simulations. The control inputs to the system are the mass flow rates into each zone and the control objective is to drive the system state into a comfort zone set (a humidity and temperature region defined on the psychometric chart). The dynamic system is shown to be passive, hence any passive controller is stabilizing and able to drive both temperature and humidity to steady states within the thermal comfort region for given ambient conditions. We then propose a set-based (passive) controller to regulate the system outputs within the comfort region. Simulation results from implementing the controller on the lumped model are then compared with CFD simulations, for a design model of an existing experimental 6-room test bed. The proposed controller design methodology is also shown to be model-independent with results of the CFD simulations verifying this feature.
This paper presents the design of a passivity-based iterative learning control (ILC) algorithm fo... more This paper presents the design of a passivity-based iterative learning control (ILC) algorithm for coupled temperature and humidity in buildings. Since buildings are subjected to repeating diur-nal patterns of disturbances, ILC algorithms can significantly improve performance. Moreover, since it is a feedforward control scheme, it can be used in conjunction with either model-free or model-based approaches such as the popular model predictive control techniques. However, model-based control is challenging for buildings because of the difficulty in identifying building thermohygrometric models. Furthermore, the control law should be designed in such a way as to address both temperature and humidity set points. We propose a model-free ILC design approach facilitated by the inherent passivity of building thermohygrometric dynamics. We first demonstrate that the building dynamics are strictly output-incremental passive. This property is then exploited to design ILC laws that guarantee convergence in the iteration domain, while being robust to model uncertainty. Since we wish to control both temperature and humidity using only one input-mass flow rate of supply air, convergence to a point is not guaranteed; instead convergence to an ellipse on the temperature-humidity plane is shown. The controller performance is demonstrated through simulation examples.
IEEE Transactions on Components, Packaging and Manufacturing Technology
This study quantitatively compares the finite element and lumped modeling techniques to analyze p... more This study quantitatively compares the finite element and lumped modeling techniques to analyze phase-change and multiphase flow in a microchannel evaporator. We compare the one-zone and two-zone models in the lumped modeling approach by considering both pressure drop and phase change in the evaporator. The study investigates the deviation in results from the lumped model relative to the finite element model for various evaporator heat loads. For the one-zone model, the relative errors in predicting the average density, pressure, temperature, and vapor quality increase with the evaporator heat load. This increase is mainly due to the nonlinear variation in the heat transfer coefficient and pressure drop, which are calculated with higher inaccuracies at larger evaporator heat loads. On the other hand, the liquid-phase region and the exit vapor quality are predicted more accurately. The overall errors using one-zone and two-zone lumped models were less than 30% when the exit qualities do not exceed 0.5. Since strategies involving flow boiling in evaporators typically avoid high exit vapor qualities to circumvent dry-out and critical heat flux, the use of lumped models under these conditions is favorable. This approach also avoids the computational costs associated with finite element models while achieving sufficient accuracy to predict the evaporator’s performance.
— This paper presents a modeling and control method for the thermohygrometric condition (temperat... more — This paper presents a modeling and control method for the thermohygrometric condition (temperature and humidity) in a multizone building. The interconnection between the zones is captured through an undirected graph. Employing an electrical circuit analogy, rooms represent capacitances, and walls and doors/windows are resistances. This model characterizes both mass and heat transfer between the zones and their coupling within each zone, extending the temperature-only resistance– capacitance models commonly used in the building control literature. By using physics-based computational fluid dynamics (CFD) simulation, we verify that this lumped-parameter model is a reasonable approximation of the physical system. The control objective is to drive the temperature and humidity of each zone into the comfort region in the psychrometric chart, using the mass-flow rate of the supplied air into the zone as the control input. In contrast to thermal-only building control, the challenge of this problem is to use a single control variable, mass-flow rate, to regulate both temperature and humidity. Our approach is to first design a feedforward control based on the desired steady-state condition within the comfort zone. We then draw on our previous work on passivity-based building temperature control to show that the thermohygrometric model around the steady state is strictly passive, from the mass-flow rate to a synthetic output combining temperature and humidity. This allows the use of any passive feedback controller combined with the feedforward to achieve robust stabilization about the desired operating point. The feedforward may be further adaptively updated, resulting in an integral-control term in the controller. Finally, to reduce the energy usage, we only apply the controller outside of the comfort zone and turn OFF the controller within the comfort zone. To illustrate the effectiveness of the proposed control strategy, simulation results using both the lumped and CFD models are presented for an existing physical six-room testbed.
Predicting and controlling the flow regime transition of multiphase fluids in microchannels is es... more Predicting and controlling the flow regime transition of multiphase fluids in microchannels is essential for various energy applications, such as flow boiling, de-emulsification and oil recovery processes. This in turn requires a better understanding of multiphase flow behaviors in microchannels with various channel surface wettability, fluid interfacial tension and flow rates. In this paper, experiments and Lattice Boltzmann method (LBM) simulations are carried out to study complicated multiphase flow at micro or meso scales. With the Shan-Chen multiphase LBM model, the flow pattern transitions of adiabatic two phase flow in a microchannel were investigated. The effects of surface wettability and liquid/gas velocity ratio on the flow regime transition were further studied. A series of two-phase flow experiments were conducted on a PDMS microfluidic device under different gas/oil velocity ratios. Under various surface wettability conditions, our simulation results agree well with the flow visualization experiments equipped with a high speed camera (HSC). Our finding shows that the cross-section meniscus curve width, corresponding to the shadow in the HSC photo, increases with decreasing contact angle, which was confirmed by the simulated liquid/gas distribution. Besides the influence of surface wettability, the role of gas/liquid velocity ratio on two-phase flow regime transition was discussed in detail. The proposed approach paves the way to probe complicated physics of multiphase flows in microporous media.
— This paper presents a modeling and control method for the thermohygrometric condition (temperat... more — This paper presents a modeling and control method for the thermohygrometric condition (temperature and humidity) in a multizone building. The interconnection between the zones is captured through an undirected graph. Employing an electrical circuit analogy, rooms represent capacitances, and walls and doors/windows are resistances. This model characterizes both mass and heat transfer between the zones and their coupling within each zone, extending the temperature-only resistance– capacitance models commonly used in the building control literature. By using physics-based computational fluid dynamics (CFD) simulation, we verify that this lumped-parameter model is a reasonable approximation of the physical system. The control objective is to drive the temperature and humidity of each zone into the comfort region in the psychrometric chart, using the mass-flow rate of the supplied air into the zone as the control input. In contrast to thermal-only building control, the challenge of this problem is to use a single control variable, mass-flow rate, to regulate both temperature and humidity. Our approach is to first design a feedforward control based on the desired steady-state condition within the comfort zone. We then draw on our previous work on passivity-based building temperature control to show that the thermohygrometric model around the steady state is strictly passive, from the mass-flow rate to a synthetic output combining temperature and humidity. This allows the use of any passive feedback controller combined with the feedforward to achieve robust stabilization about the desired operating point. The feedforward may be further adaptively updated, resulting in an integral-control term in the controller. Finally, to reduce the energy usage, we only apply the controller outside of the comfort zone and turn OFF the controller within the comfort zone. To illustrate the effectiveness of the proposed control strategy, simulation results using both the lumped and CFD models are presented for an existing physical six-room testbed.
— This paper presents a modeling and control strategy for comfort zone set-based control of tempe... more — This paper presents a modeling and control strategy for comfort zone set-based control of temperature and humidity in buildings. We first propose a coupled model for humidity and temperature dynamics based on lumped parameter analysis. The interconnection of rooms/zones is captured through an undirected graph, with rooms represented as capacitances and walls and doors/windows as resistances. Unlike traditional RC-models, however, this model captures both mass and heat transfer between zones as well as the bilinearity in the input mass flow-rate. Key parameters are identified by the model, such as mass (and thermal) conductance between zones as well as mass (and thermal) capacitance and this model structure is then validated using physics-based Computational Fluid Dynamics (CFD) simulations. The control inputs to the system are the mass flow rates into each zone and the control objective is to drive the system state into a comfort zone set (a humidity and temperature region defined on the psychometric chart). The dynamic system is shown to be passive, hence any passive controller is stabilizing and able to drive both temperature and humidity to steady states within the thermal comfort region for given ambient conditions. We then propose a set-based (passive) controller to regulate the system outputs within the comfort region. Simulation results from implementing the controller on the lumped model are then compared with CFD simulations, for a design model of an existing experimental 6-room test bed. The proposed controller design methodology is also shown to be model-independent with results of the CFD simulations verifying this feature.
This paper presents the design of a passivity-based iterative learning control (ILC) algorithm fo... more This paper presents the design of a passivity-based iterative learning control (ILC) algorithm for coupled temperature and humidity in buildings. Since buildings are subjected to repeating diur-nal patterns of disturbances, ILC algorithms can significantly improve performance. Moreover, since it is a feedforward control scheme, it can be used in conjunction with either model-free or model-based approaches such as the popular model predictive control techniques. However, model-based control is challenging for buildings because of the difficulty in identifying building thermohygrometric models. Furthermore, the control law should be designed in such a way as to address both temperature and humidity set points. We propose a model-free ILC design approach facilitated by the inherent passivity of building thermohygrometric dynamics. We first demonstrate that the building dynamics are strictly output-incremental passive. This property is then exploited to design ILC laws that guarantee convergence in the iteration domain, while being robust to model uncertainty. Since we wish to control both temperature and humidity using only one input-mass flow rate of supply air, convergence to a point is not guaranteed; instead convergence to an ellipse on the temperature-humidity plane is shown. The controller performance is demonstrated through simulation examples.
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Papers by Charles Okaeme