Abstract:
A system combines the thermal conductivity characteristics of certain solids with the high specific heat values of appropriate fluids to enhance the overall heat transfer characteristics of a heat exchanger. The system comprises a fluid channel disposed in a heat exchanger unit with a slurry as the convective heat transfer medium. The slurry comprises an appropriate fluid with field reactive particles suspended therein. Field emitters are located along the walls of the fluid channel whereby the distribution of particles within the slurry is manipulated to achieve enhanced heat transfer characteristics.

Description:
BACKGROUND  
       [0001]     Heat transfer limitations and optimization have been an engineering design constraint for decades. Thermal management has been a key consideration in the design and development of military hardware in the past century. More recently, cooling effectiveness has become a very important technical challenge and is one of the limiting factors in the further development of a range of military related disciplines including electronic, high-energy weapon and propulsion systems. Microelectronic components are particularly susceptible to thermal management problems and have become an integral component in most military systems. Many of these components cannot operate at elevated temperatures resulting in a thermal management system becoming a key consideration.  
         [0002]     Convective heat transfer is one way of addressing thermal management. Convective heat transfer is the heat transfer process that is executed by the flow of a fluid over a surface of a medium. Convective heat transfer includes advective heat transfer, which is based on the velocity of the fluid flow compared to the medium, and conductive heat transfer, which is based on static fluid adjacent to the medium. In convective heat transfer, the fluid acts as a carrier for the energy that it draws from (or delivers to) the surface of the medium.  
         [0003]      FIG. 8  illustrates a convective heat transfer system wherein a fluid  804  flows with a velocity f adjacent to a medium  802 . When fluid  804  passes adjacent to medium  802 , the portion of fluid  804  that is directly adjacent to medium  802 , i.e., at the boundary of medium  802 , has a velocity of zero as a result of a sheer stress created at medium  802 . The velocity of fluid  804  increases to a maximum as the distance from the boundary of medium  802  increases. A layer of fluid  804  adjacent to the boundary of medium  802 , referred to as the boundary layer  806 , is the layer fluid having a velocity much lower than the average velocity of the remainder, or bulk, of the flowing fluid. As illustrated in  FIG. 8 , flowing fluid  804  creates a boundary layer  806  adjacent to the boundary of medium  802  such that only a bulk of fluid  804 , or bulk fluid  808 , essentially flows.  
         [0004]     For purposes of a simplistic explanation of heat transfer in the system of  FIG. 8 , assume that medium  802  has a temperature H m , whereas bulk fluid  808  has an average temperature H f , wherein H m ≠H f . The term “average” is used to describe the temperature H f  of bulk fluid  808  because bulk fluid  808  will have many different local fluid temperatures, but the overall temperature of bulk fluid  808  can be generally described using the average of such temperatures. If H m &lt;H f , then heat will be convectively transferred from bulk fluid  808  to medium  802  through boundary layer  806 . Atlernatively, if H m &gt;H f , then heat will be convectively transferred from medium  802  to bulk fluid  808  through boundary layer  806 .  
         [0005]     There are many ways to specify the types of convection. The flow over the surface can be specified as internal, e.g., with pipes or ducts, or external, e.g., with fins. The motive force behind the bulk fluid motion can be forced, e.g., by a fan or pump, or natural, e.g., driven by buoyancy forces caused by fluid density changes with temperature. The flow can be further classified as single-phase, wherein the fluid does not change phase or multi-phase, e.g., boiling or condensation.  
         [0006]     There are many specific characteristics of the flow of a fluid that greatly affect the heat transfer rate from/to the medium&#39;s surface, but the two categories that govern the effectiveness of single-phase forced convective heat transfer are: 1) the rate of conduction of energy (heat) to/from the medium surface; and 2) the rate of conveyance of energy toward/away from the surface with the mass flow of the bulk fluid. The rate of conduction is dictated by both the thermal conductivity of the fluid and the temperature of the fluid in the boundary layer. The thermal conductivity of the fluid is a temperature dependent physical property of the fluid that is being used in the convection process. The temperature of the fluid in the boundary layer is influenced by the amount of heat transferred, the specific heat of the fluid and the flow characteristics in the boundary layer. Poor flow characteristics will not allow the fluid in the boundary layer to be replaced by the bulk fluid. The major factors that determine the rate of energy conveyance are the mass flow rate of the bulk fluid and the specific heat capacity of the fluid.  
         [0007]     The best convective heat transfer occurs when the fluid properties and flow conditions are optimized. The optimal fluid properties are high thermal conductivity and high specific heat capacity. The flow conditions that favor optimal convective heat transfer include high local fluid velocity at the medium&#39;s surface. Unfortunately, it is difficult to optimize both the thermal conductivity and specific heat capacity of a fluid, and the naturally occurring boundary layer limits the flow near the medium&#39;s surface.  
         [0008]     Two specific areas of convective heat transfer research address the fluid property and surface flow problems. These two areas include the use of nanofluids and the use of magnetic fields with magnetrohetrological fluids. Both have limited success in enhancing the rate of convective heat transfer.  
         [0009]     Nanofluids are conventional fluids with tiny particles therein that may typically be no larger than several nanometers. The particles are usually of high thermal conductivity and are added to the fluid to increase the bulk thermal conductivity of the fluid. In general, the particles are metal or metal oxides, such as for example Cu, CuO and Al 2 O 3 . A significant increase in thermal conductivity has been reported for various volume fractions of particles suspended in different fluids. Experiments performed utilizing nanofluids have shown an increase of convective heat transfer rate when compared to the same fluid without nanoparticles.  
         [0010]     The bulk majority of the research in magnetic fields used to enhance heat transfer is focused on the hydrodynamic manipulation of magnetorhetrological fluids (ferrofluids). Much of the numerical and theoretical investigation centers on a disruption of the boundary layer through the use of a constant magnetic field acting on a ferrofluid. In all of these cases the fluid is assumed to remain homogeneous in particle composition. Another area of research utilizes magnetic fields and soft magnetic particles to reduce the disadvantage of inefficient gas-solid two-phase flow. The magnetic particles are attracted to the wall, which has a temperature that is higher than the temperature of the bulk fluid flowing by the wall. The attracted particles are heated above their Curie point by thermal conduction and then are carried away by the flow. By conservation of energy, the temperature of the wall is generally decreased by an amount proportional to the amount of heat carried away, whereas the temperature of the bulk fluid is increased by an amount proportional to the amount of heat carried away.  
         [0011]     Neither the use of nanofluids nor constant magnetic fields, described above, optimize the potential for improving convective heat transfer performance.  
         [0012]     What is needed is system and method for improving convective heat transfer performance.  
       BRIEF SUMMARY  
       [0013]     It is an object of the present invention to improve convective heat transfer performance by providing combined thermal conductivity characteristics of certain solids with the high specific heat values of appropriate fluids to enhance the overall heat transfer characteristics of a heat exchanger.  
         [0014]     In order to achieve at least the above-discussed object, in accordance with one aspect of the present invention, a system for transferring heat away from the surface of a heat exchanger is presented. The system comprises a fluid channel disposed in a heat exchanger unit, containing a slurry as the convective heat transfer medium. The slurry comprises an appropriate fluid with field reactive particles suspended therein. Additionally, field emitters are located along the walls of the fluid channel to manipulate the dispersion of the particles within the slurry. By attracting the field reactive particles directly to the walls of the fluid channel, heat can be effectively transferred to/from the particles with minimal thermal resistance. Releasing the particles into the bulk fluid allows the heat to be transferred to/from the high specific heat fluid very effectively because of the large total surface area of the particles when separated within the bulk fluid. The attracting and releasing the particles from the walls of the fluid channel has the additional advantage of breaking up the boundary layer and allowing fluid from the core bulk flow to come in more direct contact with the wall.  
         [0015]     In one exemplary embodiment, the slurry comprises ferromagnetic (or other materials effected by magnetic fields) particles and a liquid used as the convective heat transfer medium. A time-varying magnetic field is produced in a fluid channel of the heat exchanger to cause the ferromagnetic particles to be attracted to the walls of the fluid channel. The field would remain energized long enough to attract the ferromagnetic particles to the walls and also allow the heat to be transferred to/from the ferromagnetic particles. The ferromagnetic particles are highly conductive and therefore are able to quickly conduct heat directly from the wall with their superior thermal conductivity, which may be as much as three orders of magnitude over common heat transfer liquids. The particles are then released back into the fluid and transfer heat into/out of the bulk liquid. With a magnetic field utilized as the particle manipulative motive force, particle removal may be enhanced by: de-energizing the magnetic field that initially attracted the particle to the wall; and then energizing of a magnetic field that is displaced spatially from the field that initially attracted the particle to the wall. Although the particles can be removed from the wall by the hydrodynamic forces of the flowing fluid, the additional use of a magnetic field would help ensure the removal of particles from the solid surface.  
         [0016]     In another exemplary embodiment, the slurry comprises particles that are affected by electric fields and a liquid used as the convective heat transfer medium. A time-varying electric field is produced in a fluid channel of the heat exchanger to cause the particles to be attracted to the walls of the fluid channel. The field would remain energized long enough to attract the particles to the walls and also allow the heat to be transferred to/from the particles. The particles are highly conductive and therefore are able to quickly conduct heat directly from the wall with their superior thermal conductivity. The particles are then released back into the fluid and transfer heat into/out of the bulk liquid. With an electric field utilized as the particle manipulative motive force, particle removal may be enhanced by: de-energizing the electric field that initially attracted the particle to the wall; and then energizing an electric field that is displaced spatially from the field that initially attracted the particle to the wall. Further, particle removal may be additionally enhanced by reversing the polarity of the electric field that initially attracted the particle to the wall. Although the particles can be removed from the wall by the hydrodynamic forces of the flowing fluid, the additional use of an electric field would help ensure the removal of particles from the solid surface.  
         [0017]     Superior heat transfer occurs because of the large surface area to volume ratio of the particles after they separate from the wall and mix back into the bulk fluid. Another benefit of the attraction and repulsion of the particles from the walls is the breaking up of the boundary layer close to the fluid channel wall. The breaking up of the boundary layer also enhances the convective heat transfer by helping to transport bulk fluid having the average bulk fluid temperature close to the surface of the fluid channel.  
         [0018]     The ferromagnetic particle size can vary, for example from millimeter to nanometer sized particles. It is important that the slurry does not remain homogeneous when acted on by a field because the particles need the ability to be individually attracted to the solid surface.  
         [0019]     Additional objects advantages and novel features of the invention are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     
    
     BRIEF SUMMARY OF THE DRAWINGS  
       [0020]     The accompanying drawings which are incorporated in and form a part of the specification, illustrate exemplary embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0021]      FIG. 1  illustrates a fluid channel containing a slurry in accordance with one embodiment of the present invention;  
         [0022]      FIG. 2  illustrates a heat transfer unit in accordance with one embodiment of the present invention;  
         [0023]      FIG. 3   a  illustrates a field created by an emitter in the fluid channel of  FIG. 1 ;  
         [0024]      FIG. 3   b  illustrates the breakup of boundary layer in the fluid channel of  FIG. 1 ;  
         [0025]      FIG. 4  illustrates a functional diagram of an apparatus that is operable to provide a consistent constant heat flux, a consistent constant temperature heat sink and a constant slurry flow rate;  
         [0026]      FIG. 5  is a more detailed illustration of apparatus of  FIG. 4 ;  
         [0027]      FIG. 6  is a functional block diagram of the control and energizing elements for electromagnets used as field emitters in the embodiment of  FIG. 5 ;  
         [0028]      FIGS. 7A-7D  are exploded views of a portion of tubing  618  and illustrate the heat transfer process in accordance with one embodiment of the present invention; and  
         [0029]      FIG. 8  illustrates a convective heat transfer system.  
     
    
     DETAILED DESCRIPTION  
       [0030]      FIG. 1  illustrates a fluid channel  102 , having a slurry  100  flowing therethrough, to be used in accordance with the present invention. Slurry  100  includes fluid  101  and particles  103  therein. Particles  103  may comprise any type of material or combination of materials that has a field-reactive property non-limiting examples of which include an electric and/or a magnetic field reactive property. Particles  103  possess greater thermal conductivity than fluid  101 . Additionally, particles  103  may vary in size and shape and further need not be uniform. The size of particles  103  preferably should not be so large as to impede the flow of slurry  100  through fluid channel  102 . In an exemplary embodiment, particles  103  are no larger than several nanometers. Further, in the presence of a field, particles  103  are capable of non-homogeneous distribution within fluid  101 .  
         [0031]     In operation, slurry  100  flows through fluid channel  102  creating a boundary layer  105  adjacent to a wall  104 . Accordingly, a bulk of the slurry or bulk slurry  107  is the portion of slurry  100  that is flowing, and flows with an average velocity V f . Wall  104  initially has a temperature t 1 , whereas the average temperature of bulk slurry  107  is t 2 . For purposes of simplifying an explanation of an embodiment of the present invention, the case will be discussed wherein t 1  is greater than t 2 , wherein slurry  100  will take heat from wall  104  (e.g., slurry  100  will have a cooling effect on wall  104 ). One of skill in the art would readily recognize the operation of the present invention in the case of t 2  being greater than t 1 , wherein slurry  100  will give heat to wall  104  (e.g., slurry will have a heating effect on wall  104 ).  
         [0032]      FIG. 2  illustrates a heat transfer unit  208  in accordance with one embodiment of the present invention. As depicted in  FIG. 2 , fluid channel  102  containing slurry  100 , and field emitters  106 ,  108 ,  110  and  112  displaced along fluid channel wall  104 , are disposed in heat transfer unit  208  that is capable of maintaining a temperature t 1  that is greater than the temperature t 2  of a bulk of slurry  100 . Additionally  FIG. 2  depicts a power supply  202  and signal generator  203  that provides a signal to control the activation of field emitters  106 ,  108 ,  110  and  112 . Convective heat transfer occurs between heat transfer unit  208  and slurry  100 , as slurry  100  moves through fluid channel  102 .  
         [0033]     As slurry  100  moves through fluid channel  102 , fields are emitted by emitters  106 ,  108 ,  110  and  112 . The fields attract particles  103  to the walls of fluid channel  102  in the proximity of emitters  106 ,  108 ,  110  and  112 . The presence of particles  103  adjacent to or in close proximity to fluid channel wall  104  disrupts the boundary layer and enables increased convective heat transfer to slurry  100  across fluid channel wall  104  than would otherwise be possible in the absence of particles  103 . Particles  103  are then released from the close proximity to fluid channel wall  104  and disperse throughout fluid  101  to transfer the recently acquired heat to fluid  101 . The transferred heat is then carried away from a portion of heat transfer unit  208  as slurry  100  moves through fluid channel  104 .  
         [0034]     Returning to  FIG. 1 , field emitters  106 ,  108  and  110 , are displaced along the sides of fluid channel wall  104 . Each field emitter can emit a field to affect particles  103 . When field  302  is emitted, as depicted in  FIG. 3A , particles  103  are attracted to fluid channel wall  104  in the proximity of emitter  106 . By attracting particles  103  directly to the surface of fluid channel wall  104 , thermal resistance of slurry  100 , caused by the distance between particles  103  and fluid channel wall  104 , is reduced and more effective heat transfer to particles  103  is achieved. Because particles  103  have greater thermal conductivity than fluid  101 , total heat transfer achieved is greater than would otherwise be possible in the absence of particles  103 .  
         [0035]      FIG. 3B  illustrates the separation of particles  103  from fluid channel wall  104  as emitter  106  stops emitting field  302 . When emitter  106  stops emitting, particles  103  are released from fluid channel wall  104  and disperse within fluid  101 . When particles  103  disperse within fluid  101 , heat absorbed by particles  103  while adjacent to fluid channel wall  104  is transferred to fluid  101  and carried away in the direction of V f . Additionally, the release of particles  103  from fluid channel wall  104  further enhances heat transfer by breaking up boundary layer  105  adjacent to fluid channel wall  104 , allowing more of bulk slurry  107  having temperature t 2  to come in direct contact with fluid channel wall  104 .  
         [0036]     Returning to  FIG. 2 , in an exemplary embodiment of the present invention, signal generator  203  alternately activates field emitter parts  106  and  110  and then  108  and  112 . In this way, after particles  103  are released from one portion of fluid channel wall  104  in the proximity of one of emitters  106  and  110 , they are subsequently attracted to another portion of fluid channel wall  104  in the proximity of another one of emitters  108  and  112  spatially displaced down stream along fluid channel wall  104 . Accordingly, particles  103  absorb heat while attached to fluid channel wall  104 , then transfer that heat to fluid  101  as they are released from a portion of fluid channel wall  104  and disperse within fluid  101 . More heat is then absorbed as particles  103  are attracted to another portion of fluid channel wall  104  as slurry  100  flows down stream through fluid channel  102  and another emitter emits a field  302  attracting particles  103  to a portion of fluid channel wall  104 .  
         [0037]     A working embodiment of a convective heat transfer system will now be described in detail with reference to  FIGS. 4-7D .  
         [0038]      FIG. 4  illustrates a functional diagram of an apparatus  400  that is operable to provide a consistent constant heat flux, a consistent constant temperature heat sink and a constant slurry flow rate. Apparatus  400  is further operable to provide an accurate comparison between the maximum temperature of the constant flux heat source during a first state of operation wherein there is no applied magnetic field and during a second state of operation wherein there is an applied alternating magnetic field. Apparatus  400  is additionally operable to provide an accurate comparison between the increase in the amount of heat flux between the first state of operation and the second state of operation, when the maximum temperature of the heated tube is kept constant.  
         [0039]     A slurry is heated in tubing  402 , exits tubing  402  and enters a pump  404 . After exiting pump  404 , the slurry enters a heat exchanger  406 . Heat is removed from the slurry, while in heat exchanger  406 , by using chilled water provided by a recirculating chiller  408 .  
         [0040]      FIG. 5  is a more detailed illustration of apparatus  400 . The slurry is contained within the tubes  502  and is cooled by chilled water provided by tube  504  from recirculating chiller  408 . After being cooled in heat exchanger  406  the slurry flows to inlet  506  of vertically mounted tubing  508 . A vertical orientation, with flow from top to bottom, was utilized for both tubing  508  and heat exchanger  406  to minimize particle-settling effects of gravity. In a working embodiment, a pumping system was utilized that included a variable speed two head rotoflex pump to minimize problems associated with particle-laden flows.  
         [0041]     The electrical components of the system of  FIG. 5  may be grouped into three general areas: electromagnet control, heater control and instrumentation. The functional block diagram in  FIG. 6  shows the control and energizing elements for electromagnets used as field emitters in the embodiment of  FIG. 5 . A signal, for example a square-wave signal, is initially generated in a signal generator  602 , which allows changing of frequency of the wave. The signal is inputted into an amplifier/splitter  604 , which amplifies and splits the signal into two channels. Channel C 1  is inverted to be 180 degrees out of phase with channel C 2 . Each channel is fed to a separate respective power amplifiers  606  and  608 . Each power amplifier  606  and  608  powers a pair of U-shaped electromagnets  610  and  612 , and  614  and  616 , respectively, mounted on each side of tubing  618 . Electromagnets  610 ,  612 ,  614  and  616  create magnetic field lines oriented generally perpendicular to the wall of tubing  618 . This orientation causes the particles to be attracted to and to agglomerate near the end of the electromagnet in a direction that is generally perpendicular to the surface of wall  618  and then to fan out along the lines of magnetic flux.  
         [0042]     Returning to  FIG. 5 , in an exemplary operation, the heat flux was maintained at a constant value by two heater elements  510  and  512  placed approximately 40% and 60% down the length of vertically mounted tubing  508 . The invention is not limited to two heaters elements, wherein any number of heating elements may be used. Further, the invention is not limited to the particular placement of the heater elements. Heater elements  510  and  512 , in one embodiment, may be connected in parallel to a power amplifier, which maintains a constant voltage and current. A plurality of heat detecting devices and instrumentation were used to determine temperature.  
         [0043]      FIGS. 7A-7D  are exploded views of a portion of tubing  618  and illustrate the heat transfer process in accordance with one embodiment of the present invention. In  FIGS. 7A-7D , H 1 (t) represents the temperature of the medium outside tubing  618 , whereas H 2 (t) represents the average temperature within tubing  618 .  
         [0044]      FIG. 7A  illustrates tubing  618  at time t=t 0   + , where H 1 (t 0   + )&gt;&gt;H 2 (t 0   + ). In the figure, coiled wires  706  and  708  of electromagnet  616  are provided with a signal from amplifier/splitter  604  via amplifier  608  to energize the core of electromagnet  616  thereby generating magnetic field lines  714  (and wherein at time t=0 the magnetic field lines are not present). As illustrated in the figure, magnetic field lines  714  pass through the wall of tubing  618  and into the slurry  702 .  
         [0045]      FIG. 7B  illustrates tubing  618  at time t=t 1 , wherein magnetic field lines  714  attract ferromagnetic particles  704  into areas  716  and  717 . At this point in time, the rate of heat transfer is increased near areas  716  and  717  as a result of the heat transfer properties of ferromagnetic particles  704 . Accordingly, heat is transferred from outside of tubing  618  to slrry  702 . As such the overall temperature of the area outside tubing  618  decreases, whereas the overall average temperature of slurry  702  increases, wherein where H 1 (t 1 )&gt;H 2 (t 1 ).  
         [0046]      FIG. 7C  illustrates tubing  618  at time t=t 2 , wherein coiled wires  706  and  708  of electromagnet  616  are no longer provided with a signal from amplifier/splitter  604  via amplifier  608  to energize the core of electromagnet  616  but coiled wires  710  and  712  of electromagnet  614  are provided with a signal from amplifier/splitter  604  via amplifier  606  to energize the core of electromagnet  614 . Accordingly, magnetic field lines  714  are no longer present but magnetic field lines  718  are generated. As illustrated in the figure, magnetic field lines  718  pass through the wall of tubing  618  and into the slurry  702 . Particles  720  leave areas  716  and  717  and begin to disperse into slurry  702 . At this point, the heat collected by ferromagnetic particles  704  is transferred into the liquid of slurry  702  such that the temperature of ferromagnetic particles  704  decreases and the temperature of the liquid of slurry  702  increases.  
         [0047]      FIG. 7D  illustrates tubing  618  at time t=t 3 , wherein magnetic field lines  718  attract ferromagnetic particles  704  into areas  722  and  724 . At this point, the rate of heat flux transfer is again increased as a result of the heat transfer properties of the now cooled ferromagnetic particles  704 . Accordingly, more heat is transferred from outside of tubing  618  to slurry  702 . As such the overall temperature of the area outside tubing  618  decreases further, whereas the overall average temperature of slurry  702  increases further, wherein where H 1 (t 3 )&lt;H 2 (t 3 ).  
         [0048]     Several data collection runs were performed with apparatus  400  with experimental parameters varied. In some exemplary data collection runs, the slurry comprised oil with iron fillings dispersed therein. The parameters that were varied included concentration of iron fillings to oil, frequency of electromagnet power and current used to energize electromagnets  610 ,  612 ,  614  and  616 . The results obtained with a constant heat input showed a dramatic decrease in the temperature of the surface of tubing  508  at the midpoint between the inlet and the outlet (the maximum pipe temperature). During these experimental runs, recirculating chiller  408  temperature maintained a temperature of 20° C. and the power inputted into the heaters was 65 W. For one of the more extreme cases, the maximum temperature of tubing  508  (at the tubing midpoint) was measured at 57.2° C., whereas with electromagnets  610 ,  612 ,  614  and  618  being deenergized and energized with a time varying square wave, the temperature measured was 42.5° C. Null checks were performed with pure mineral oil to ensure that the instrumentation was not reading incorrectly or the heaters operating improperly because of the fluctuating magnetic fields.  
         [0049]     To determine the magnitude of convective heat transfer increase, an experiment was run to determine the amount of heat that could be added and still maintain the same temperature as the full power case with electromagnets  610 ,  612 ,  614  and  616  cycling. The maximum increase in heat transfer rate that was obtained with the experimental setup was 80%.  
         [0050]     In another exemplary embodiment, an electric field may be used instead of a magnetic field to attract and repel the particles to/from the solid surface. With an electric field the highly thermal conductive particle may comprise, for example, an ionic material or some material that can be manipulated by an electric field. With an electric field utilized as the particle manipulative motive force, particle removal may be enhanced by: de-energizing the electric field that initially attracted the particle to the wall; and then energizing of an electric field that is displaced spatially from the field that initially attracted the particle to the wall. Further, particle removal may be additionally enhanced by reversing the polarity of the electric field that initially attracted the particle to the wall. Although the particles can be removed from the wall by the hydrodynamic forces of the flowing fluid, the use of a separate electric field would help ensure the removal of particles from the solid surface.  
         [0051]     In another embodiment, the orientation of the line of magnetic flux used to attract the ferromagnetic particles to the wall is varied in any direction, for example parallel to the wall of tubing  508 , such that the magnetic flux lines attract the ferromagnetic particles to the surface of tubing  508 .  
         [0052]     The foregoing description of various preferred embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments, as described above, were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.