BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus and method for enhancing heat transfer in a heat exchanger. 
     Heat exchangers typically involve a fluid flowing in a conduit and the exchange of heat between the fluid and the conduit. For example, chemical process plants typically use shell and tube-type heat exchangers to provide heat exchange between a fluid and a conduit. 
     In the design of heat exchangers, it is well known that heat transfer between a fluid flowing along a heat exchanger surface or conduit is confined primarily to a layer of fluid in contact with the heat exchanger surface. Previous attempts to enhance heat transfer include fin structures extending from the heat exchanger surface and contacting the fluid to set up a flow disturbance which prevents the stratifying or laminar flow of the fluid flowing against the heat exchanger surface. The fins typically are formed to contact the heat exchanger surface and provide higher conductive heat transfer from the fluid to the surface. 
     An insert device known as a turbulator has been employed in heat exchangers to provide a turbulent flow of the fluid against the inside surface of the conduit or tube in which the fluid is flowing. The turbulator in the tube improves heat transfer primarily by slowing down the velocity of the fluid flowing through the central portion of the tube or pipe cross section, and further improves the temperature distribution of the fluid in the cross section of the tube or conduit by conduction and mixing. 
     It is known that heat transfer applications at high temperatures involve a radiation of heat transfer which takes on a dominant influence over convection and conductive heat transfer. Attempts have been made to take advantage of higher radiation heat transfer by providing reradiant inserts. An example of a reradiant insert would be a gas recuperator as is disclosed in Kardas et al U.S. Pat. No. 3,886,976. The Kardas insert uses a floating extended surface which provides an additional area for accepting heat by convection and radiation from the hot gas in the recuperator, the Kardas insert not being integrally connected with the original heat receiving surface. Heat is retransmitted to the intended heat transfer surface by a continuous spectrum of Stefan-Boltzmann radiation. The Kardas et al patent discloses that radial mixing and large effecting radiating area can be obtained by using multileaf reradiators of the type shown in the Kardas patent in FIG. 5. 
     However, the aforementioned fins, turbulators, and recuperators have a major drawback in that these devices require a significant pressure drop through the conduit. Further, the aforementioned turbulators and fins are designed for lower temperature operation and do not produce the most efficient heat exchange insert at higher temperatures. 
     It is an object of the present invention to provide heat exchanger apparatus and method for enhancing heat exchange between a fluid and a heat exchanger surface such as a heat exchanger tube or conduit. 
     It is another object of the present invention to provide heat exchanger apparatus and method of enhanced efficiency at higher temperature differences between the fluid and heat exchanger surface. 
     It is yet another object of the present invention to provide heat exchanger apparatus and method of enhanced efficiency requiring a minimum pressure drop through the heat exchanger. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, heat exchanger apparatus and method are provided for enhancing the heat transfer between a fluid and tubular heat exchanger surface. The heat exchange apparatus of the present invention includes a tubular heat transfer surface, means for passing a heat transfer fluid along the surface, and a brush heat exchange insert positioned to impinge the fluid flowing within the heat transfer surface. In one aspect, the brush heat exchange insert is composed of a ceramic material having a high absorptance and emittance. 
     The method of the present invention includes establishing the heat transfer insert of the present invention of a wire brush insert positioned in a tube or channel to impinge the flow of a heat exchanger fluid on the surface of the insert and to enhance the heat exchange between the fluid and the heat exchange surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a and FIG. 1b depict cross-sectional views of heat exchanger tubes including an insert according to the present invention. 
     FIG. 2 shows a graphical comparision of heat transfer for gas flow parallel to wires compared to flow normal to wires. 
     FIG. 3 depicts a graphical correlation of heat transfer coefficients between the heat exchange insert of the present invention and prior art inserts. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1a, an elevational view of a cross section of pipe 1 is depicted. Heat exchange insert 2 is provided in pipe 1. Heat exchange insert 2 as depicted in FIG. 1a can be viewed as the longitudinal end view of a wire brush having core 3 and appendages 4. All of the appendages 4 are not required to contact the inside wall of the heat exchanger surface inside wall 6 of the tube or conduit 1 as will be explained hereinafter. 
     Referring to FIG. 1b, pipe 1 is shown in a cross-sectional side view. 
     The heat exchange inserts of the present invention as depicted as insert 2 in FIG. 1a and FIG. 1b have the shape substantially similar to a wire brush. 
     We have found that the number of bristles on the brush should be at least 50 per linear of brush. An insert having less than 50 bristles per linear inch provides a less efficient operation in heat transfer efficiency. On the other hand, the bristles should not be more than 500 bristles per linear inch or a pressure drop required to operate the heat exchange apparatus will be excessive. 
     The heat exchange insert of the present invention improves flow through a heat exchanger conduit and reduces pressure drop over prior art inserts such as turbulators. 
     It has been found unexpectedly that the wires of the present invention transfer more heat from a gas to a heat exchanger surface than is transferred with heat exchange inserts such as panels. An attempt at explaining why this occurs is as follows. 
     Heat transfer involves three fundamental mechanisms: conduction, convection, and radiation. Conduction involves heat transfer from one location of a unit mass to another location of the same unit mass or from a first unit mass to a second unit mass in physical contact with the first without significant movement of the particles of the unit&#39;s mass. Convection involves heat transfer from one location to another location within a fluid, either gas or liquid, by mixing within the fluid. Natural convection involves motion of the fluid from density differences attributable to temperature differences. Forced convection involves motion in the fluid set up by mechanical work applied to the fluid. At low forced velocities in the fluid, density and temperature differences are more important than at higher forced velocities. Radiation involves the heat transfer from one unit mass to another unit mass not contacting the first. Radiation takes place through a wave motion through space. 
     Heat transfer by conduction can be described by a fundamental differential equation known as Fourier&#39;s Law: ##EQU1## wherein dQ/dθ (quantity per unit time) is heat flow rate; A is area at right angles to the direction of heat flow; and -dt/dx is temperature change rate with respect to distance in the direction of heat flow, i.e., temperature gradient. The thermal conductivity is defined by k, which is dependent on the material through which the heat flows and further is dependent on temperature. Convective heat transfer involves a coefficient of heat transfer which is dependent on characteristics of fluid flow. Turbulent flow of a fluid past a solid sets up a relatively quiet zone of fluid, commonly called a film in the immediate vicinity of the surface. Approaching the wall from the flowing fluid, the flow becomes less turbulent and can be described as laminar flow near the surface. The aforementioned film is that portion of the fluid in the laminar motion zone or layer. Heat is transferred through the film by molecular conduction. In this latter aspect, light gases have the most resistance to heat transfer through the film and liquid metals have the least resistance through the laminar film region. The equation for describing heat transfer from the flowing fluid to the surface is set forth as follows in equation (2): 
     
         Q=hAΔT                                               (2) 
    
     wherein 
     Q=Quantity of heat transferred per unit time Btu/hr. 
     h=Coefficient of heat transfer=quantity of heat Btu/(hrft 2  ° F.) transferred per unit area and unit time per unit of temperature difference across the film. 
     A=Area-ft 2   
     T=Temperature difference between the gas and surface -° F. 
     Thermal radiation heat transfer involves  excitation pulse. The first spin echo occurs a duration A after the first refocusing pulse or 2A after the excitation pulse 50. The second refocusing pulse 54 is applied a duration B after the first spin echo or a duration 2A+B after the excitation pulse. The second refocusing pulse is followed by the second spin echo a duration B later, i.e. a duration 2A+2B after the excitation pulse. The third refocusing pulse 56 is applied a duration C after the second spin echo and is followed by the third spin echo 66 a duration C later, i.e. 2A+2B+2C after the excitation pulse. The fourth refocusing pulse 58 is applied a duration D after the third spin echo and is followed by the fourth spin echo 68 a duration D later, i.e. 2A+2B+2C+2D after the excitation pulse. Additional refocusing pulses are likewise followed by additional spin echoes. The data acquisition interval is not longer than the shortest of durations A, B, C and D. 
     Any radio frequency pulse can be resolved into three components: (1) a 0° component, (2) a 90° component, and (3) a 180° component. A perfect 180° pulse has only a 180° component and no 0° or 90° components. An echo occurs after a 180° rotation or refocusing pulse, provided that there is transverse magnetization in the xy plane beforehand. The 180° inversion may be accomplished with either a single 180° pulse or a pair of 90° pulses separated in time. A pair of separated 90° pulses which are treated as a 180° pulse may have a further 0° or 180° rotation therebetween. After a 180° rotation or inversion, transverse magnetization that was in phase a given duration before the refocusing pulse forms an echo that same duration after the refocusing pulse. 
     With particular reference to FIG. 2, three parasitic echoes are predicted following the second refocusing pulse. These three parasitic echoes are attributable to a 0° component of a less than perfect 90° excitation pulse 50, the 0° and 90° components of a less than perfect first refocusing pulse 52, and the 90° component of a less than perfect second refocusing pulse 54. 
     With reference first to FIG. 2A, the magnetization vectors which are rotated into the transverse xy plane by the excitation pulse 50 are in part rotated out of the xy plane by the unwanted 90° component of the imperfect 180° first refocusing pulse 52. This rotates the magnetization vectors in part back into the longitudinal or z axis which freezes their dephased state. The magnetization vectors remain 90° out of the transverse plane until the 90° component of the imperfect second refocusing pulse 54 rotates them another 90° back into the xy plane, completing the 180° inversion. The magnetization vector now commences rotating in the xy plane in the opposite direction. The magnetization vectors, having dephased for the duration A before the dephased state was frozen, now require the duration A to come back into alignment forming a first parasitic echo 80 the duration A after the second refocusing pulse, i.e. the duration 3A+B after the excitation pulse. 
     With reference to FIG. 2B, the 0° component of the excitation pulse 50 leaves some magnetization vectors along the z axis. The 90° component of the first refocusing pulse 52 rotates additional magnetization vectors from alignment with the z-axis into the transverse xy plane. Following free induction decay, these magnetization vectors commence dephasing. When the second refocusing pulse 180° is applied the duration A+B later, these vectors commence rephasing, forming a second parasitic echo 82 the duration A+B after the second refocusing pulse, i.e. at a duration 3A+2B after the first excitation pulse 50. 
     With reference to FIG. 2C, the 0° component of the first refocusing pulse 52 allows a portion of the dephasing magnetization vector components to continue dephasing until the second refocusing pulse is applied. A duration 2A+B later, the second refocusing pulse 54 rotates these magnetization components 180° for the first time. A duration of 2A+B after the second refocusing pulse, these components come back into alignment causing a third parasitic echo 84. 
     By analogously following the magnetization vectors through the 0°, 90°, and 180° components of these and any additional refocusing pulses, the location of additional parasitic echoes is analogously determined. For example, in FIG. 3, additional parasitic echoes occur at 2A+3B+C, 3A+2B+C, 3A+3B+C, 2A+3B+2C, 4A+3B+C, and so forth. Moreover, each parasitic echo is refocused by subsequent refocusing pulses causing additional parasitic echoes. 
     It is to be appreciated that in the prior art Carr-Purcell sequence in which durations A, B, C, and D were equal, the first parasitic echo 80 occured concurrently with the second spin echo 62. The second parasitic echo 64 occured concurrently with a third refocusing pulse. The third parasitic echo occured concurrently with the third spin echo. Analogously, additional parasitic echoes occurred concurrently with the later spin echoes and refocusing pulses. 
     For the spin echoes to be collected cleanly, the durations A and B are selected such that the parasitic echoes occur further from the spin echo than the data acquisition interval 70. 
     It is to be appreciated that numerous relative times for durations A, B, C, D, etc. may be selected. In the two refocusing pulse sequence illustrated in FIG. 2, the duration A is equal to the data acquisition interval and the duration B is equal to three times duration A. As another example, in a three refocusing pulse sequence, the duration A may equal twice the data acquisition interval, the duration may B equal the data acquisition interval, and duration C may equal four times the data acquisition interval. Other operative configurations include duration A being the same as the data acquisition interval, duration B being three times the data acquisition interval, and duration C being twice the data collection interval. In yet another three refocusing pulse embodiment, duration A is twice the data acquisition interval, duration B is three times the data acquisition interval, and duration C is four times the data acquisition interval. Numerous other ratios between durations A, B, and C may advantageously be employed, including fractional ratios. 
     In the four echo sequence illustrated in FIG. 3, the duration A is twice the data acquisition interval, the duration B is equal to the data acquisition interval, the duration C is four times the data acquisition interval and duration D is seven times the data acquisition interval. Yet another operative four echo sequence has the duration A equal to the data acquisition interval, the duration B three times duration A, the duration C twice duration A, and duration D six times duration A. 
     With reference to FIG. 4, an echo occurs at a time when the integrated areas under the gradients on either side of the refocusing pulse are equal. This holds true for both spin echoes and parasitic echoes in multiple echo sequences. That is, the second spin echo 64, for example, occurs when the integrated area of the gradient pulse following the second refocusing pulse 54 equals the integrated area of the gradient between the first spin echo 62 and the second refocusing pulse 54. By manipulating the gradient areas, the parasitic echoes can be moved away from the spin echoes. The manipulation of the area under the gradient can move the parasitic echoes away from the spin echoes while preserving the time symmetry of a conventional Carr-Purcell sequence in which all refocusing pulses are equally spaced. For example, increasing or decreasing the gradient amplitude accelerates or retards the occurance of the forecast echo. 
     The gradient strength during data acquisition varies with the length of the acquisition window, the data sampling rate, and the requisite field of view. The gradient strength is the same for the acquisition of each spin echo in the train for simplicity of processing the collected data. The actual amplitude or value of the gradient between sampling durations is not significant to the collected data, provided that the gradients areas in each inter-event interval are adhered to. The asymmetric gradient profiles enable the parasitic and spin echoes to be shifted apart, even while implementing a conventional Carr-Purcell symmetric refocusing pulse sequence. By appropriately tailoring the gradient pulses and appropriately positioning the refocusing pulses, the spin echoes and the parasitic echoes can be positioned at convenient times for a pathology to be observed. 
     With continuing reference to FIG. 4, a first read gradient pulse 90 is applied between the excitation pulse 50 and the first refocusing pulse 52. A second read gradient pulse 92 is applied subsequent to the first refocusing pulse. When the integrated area under the second read gradient pulse 92 equals the amplitude of the first gradient pulse 90 integrated over its duration, the first spin echo 62 occurs. The amplitude of the read gradient is held constant during the data acquisition interval 70. After the first spin echo, a third read gradient pulse 94 is applied. The third read gradient pulse 94 is continuous with the second read gradient pulse 92 and, for purposes of definition herein, is considered to commence at the first spin echo 62. 
     After the second refocusing pulse 54, a fourth read gradient pulse 96 is applied. When the area under the fourth read gradient pulse is equal to the area under the third read gradient pulse, i.e. when the amplitude of the fourth read gradient integrated with respect to time equals the amplitude of the third read gradient pulse integrated with respect to its duration, the second spin echo 64 occurs. For simplicity in handling the collected data, the amplitude of the read gradient during the second spin echo data acquisition interval is the same as the amplitude of the read gradient during the first spin echo data acquisition interval. 
     A fifth read gradient 98 follows continuously on the fourth read gradient. By definition, the fourth read gradient ends and the fifth read gradient commences simultaneously with the second spin echo. A sixth read gradient pulse 100 is applied after the third refocusing pulse 56. When the amplitude of the sixth gradient pulse integrated with respect to duration equals the amplitude of the fifth read gradient pulse integrated with respect to its duration, the third spin echo 66 occurs. The read gradient 102 is continued beyond the third spin echo for at least the data acquisition duration 70. 
     FIG. 5 illustrates an asymmetric read gradient sequence in conjunction with a refocusing pulse sequence with asymmetric timing. The first spin echo 62 again occurs when the amplitude of the second read gradient 92 integrated with respect to time equals the amplitude of the first read gradient 90 integrated with respect to time. The second spin echo 64 occurs when the amplitude of the fourth read gradient 96 integrated with respect to time is equal to the amplitude of the third read gradient 94 integrated with respect to its duration. The third spin echo 66 occurs when the amplitude of the fifth read gradient 100 integrated with respect to time equals the amplitude of the fourth read gradient 98 integrated with respect to its duration. All gradient pulses are held at the same preselected amplitude during the data acquisition interval 70. Optionally, the sequence may be continued for additional inversions, rotations, and other magnetization manipulations. 
     In the illustrated sequence, each pair of the read gradients are configured symmetrically about the intervening refocusing pulse. This read gradient symmetry preserves the spin echo refocusing pulse relationships discussed in conjunction with FIGS. 2 and 3, above. However, the read gradient amplitude variations between the data acquisiton intervals and the refocusing pulses are selected to move the parasitic echoes away from the spin echoes. For example, during the increased amplitude fifth read gradient 98, parasitic echoes which follow the second refocusing pulse 54 are advanced, i.e. occur closer to the second refocusing pulse. The advancement tends to condense the parasitic echoes and expand the parasitic echo free regions. 
     As another alternative sequence, the refocusing pulse can be displaced in time from the center of the interval between the spin echoes or between the initial excitation pulse 50 and the first spin echo 62 of FIGURE 5. This allows a controlled expression of chemical shift differences between distinct materials in the subject. More specifically, the interval between the first refocusing pulse 52 and the second gradient pulse 92 can be increased by a duration in which the magnetizations of water and fat become 180° out of phase. This emphasizes boundary definition in the resultant image. Alternately, separate water and fat images might be recovered. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as including all such alterations and modifications in so far as they come within the scope of the appended claims or the equivalents thereof.
The present invention provides heat exchanger apparatus and method including a tubular heat transfer surface, a heat transfer fluid which is made to pass along the surface, and a wire brush heat exchanger insert positioned to impinge the fluid flowing within the heat transfer surface. The brush heat exchange insert is composed of a ceramic material having a high absorptance and emittance.